ALBERT

Notes: With the input of order 10–20 000 cm−1 of vibrational energy, the hydrogen atoms in small gas-phase molecules such as HCN, HCP, and acetylene can undergo internal rotation about the heavy-atom core (CN–CP–CC), breaking and reforming covalent bonds in the process. This article investigates the quantum and classical dynamics of covalent bond-breaking internal rotation, particularly the vibrational energy flow between the hindered internal rotor mode and a stretch mode. The aim is to relate polyad effective Hamiltonian techniques, which have been highly successful in the analysis of high overtone spectra, to the theory of isomerization rates. That is, as approximate constants of motion, polyad numbers constrain vibrational energy flow, and we investigate the extent and mechanism of their breakdown due to nascent bond-breaking internal rotation. Our simple model consists of a spherical pendulum coupled to a harmonic oscillator, which admits a number of analytical results. The central conclusion is that polyad breakdown is a generic consequence of higher order resonances induced by a saddle point but is far from complete, in the sense that the majority of states with energies close to the saddle point can continue to be labeled with polyad numbers; only those with substantial probability density close to the saddle point itself no longer belong to moderately well defined polyads. Our model is particularly relevant to the vibrational structure of HCP, the polyad structure of which has been well studied up to ∼19 000 cm−1; our model predicts systematic polyad breaking at higher energies. © 2001 American Institute of Physics.

Notes: The cross section and asymmetry parameter profiles of (C6H6)2Cr have been calculated with a method based on the density-functional theory with an explicit treatment of the continuum wave function, with a single center basis set of B-spline functions and with an exchange-correlation potential with the correct Coulomb asymptotic behavior. The method has proven efficient from the outer valence states up to the metal and carbon core. The results are in good agreement with the available experimental data, and suggest that important features, not yet measured, should be present in the high-energy range of the valence and core profiles. © 2001 American Institute of Physics.

Notes: Liquid hydrogen fluoride is a well-known hydrogen bonded substance, in many aspects related to liquid water, and for which a wide variety of interaction models have recently been proposed. We have studied two of these models by means of a reference hypernetted chain equation in order to assess the ability of this latter approach to describe the properties of this highly associative system. Our calculations, when compared with molecular dynamic results, show that the integral equation reproduces quantitatively both the structure and the thermodynamics of liquid hydrogen fluoride over a wide range of thermodynamic states. However, the integral equation approach is apparently unable to produce estimates for the phase diagram since the low-density (gas phase) side of the binodal curve lies inside the nonsolution region of the equation. This failure can be understood as the result of the inability of standard integral equation theories to account for the behavior of low density strongly associative systems like highly charged electrolytes or, in this case, the gaseous phase of hydrogen fluoride. © 2001 American Institute of Physics.

Notes: A model for the description of the electronic ground state of the triiodide ion in solution is developed. It is based on the "diatomics in molecules" technique and is parametrized from experimental data. The solvent molecules are treated by classical intermolecular potentials. The solvent–ion interaction, which depends on the instantaneous positions of the solvent molecules, enters into the Hamiltonian matrix elements as a spatially varying external electrostatic potential. We use the model to investigate the distribution of the bond lengths of a linear triiodide ion in water at 300 K using Monte Carlo calculations. We find that under these conditions the molecule is significantly distorted with considerable redistribution of charge and bond lengths of 2.95 Å and 3.38 Å. The free energy barrier to switching bond lengths at room temperature is quite high (of the order of 10 kT) so that the distortion is predicted to have a long lifetime. The distribution of instantaneous vibrational frequencies is investigated and shows that the solvent has a greater effect on the frequency of the antisymmetric stretch than on that of the symmetric stretch vibration. © 2001 American Institute of Physics.

Notes: The structure of a uniform simple liquid is related to that of a reference fluid with purely repulsive intermolecular forces in a self-consistently determined external reference field (ERF) φR. The ERF can be separated into a harshly repulsive part φR0 generated by the repulsive core of a reference particle fixed at the origin and a more slowly varying part φR1 arising from a mean field treatment of the attractive forces. We use a generalized linear response method to calculate the reference fluid structure, first determining the response to the smoother part φR1 of the ERF alone, followed by the response to the harshly repulsive part. Both steps can be carried out very accurately, as confirmed by computer simulations, and good agreement with the structure of the full Lennard-Jones fluid is found. © 2001 American Institute of Physics.

Notes: We investigated the kinetics of thermal formation of anion vacancies and the subsequent stoichiometry changes on (110) cleavage surfaces of III–V semiconductors by scanning tunneling microscopy. We found that the rate of spontaneous formation of monovacancies depends very sensitively on the doping of the underlying semiconductor and the concentration of surface vacancies. It is shown that the position of the Fermi energy at the surface is the major electronic influence on the energy barrier height for the vacancy formation. We found barrier heights in the range of 1.1–1.3 eV for GaAs and InP. The physical factors affecting the vacancy formation and the surface stoichiometry are discussed in detail. © 2001 American Institute of Physics.

Notes: Results are presented for the reaction of gas-phase H atoms with H atoms adsorbed onto a variety of substrates. Time-dependent quantum methods are used to compute reaction cross sections and product H2 rotational and vibrational distributions for a large number of model potential energy surfaces. The potentials which model reactions on metals exhibit a wide range of reactivity. In addition, the single-collision Eley–Rideal reaction cross sections are generally small, suggesting that hot-atom processes should in general play an important role in H2 formation on metal surfaces. These observations are consistent with recent experiments. Eley–Rideal reactivity is shown to increase as the strength of the H-substrate bond decreases, and H atom trapping becomes less favorable. The cross sections for the reaction of H(g) with H adsorbed onto model graphite surfaces are generally large (5–10 Å2). © 2001 American Institute of Physics.

Notes: A comparison between experiment and theory is performed for the scattering of (v=1, j=1) H2 from Cu(100) at normal incidence. Experimentally, this system was studied using molecular beam techniques, with stimulated Raman pumping employed to overpopulate (v=1, j=1) in the incident beam, and resonance enhanced multi-photon ionization used to detect the H2 scattered in two (v=1, j) states, and two (v=0, j) states. Theoretically, six-dimensional wave packet calculations were performed, employing a new, extended potential energy surface that was computed with density functional theory, using the generalized gradient approximation and a slab representation of the metal surface. Theory and experiment are in good agreement for the survival probability, i.e., the probability for rovibrationally elastic scattering. However, the theory overestimates the probabilities for rotationally inelastic scattering (to v=1, j=3) and for rovibrationally inelastic scattering (to v=0, j=5 and 7) for channels that could be determined experimentally. The cause of these discrepancies is discussed, as are possibilities for future improvements in the theory as well as the experiment. © 2001 American Institute of Physics.

Notes: The vibrational modes of CO adsorbed on Pt(111) in the c(4×2) structure have been studied within the harmonic approximation, using density functional calculations. The characters, fundamental energies, and dipole activities have been determined for all modes. For top-adsorbed molecules, the static adsorbate–adsorbate interaction is found to induce energy splitting among frustrated lateral translational modes, which have previously been assumed to be degenerate, and a reassignment of previously measured vibrational energies to low-energy modes is proposed. For bridge-adsorbed molecules, the frustrated rotational fundamental transitions, which should be dipole forbidden from the local adsorption site symmetry, are found to be weakly dipole active. © 2001 American Institute of Physics.

Notes: The aggregation of silanized glass spheres (75±5 μm diam) was studied experimentally at liquid–air (water–air, aqueous surfactant solution–air, and aqueous glycerol solution–air) interfaces from a kinetic point of view. The number, the size, and the polydispersity of clusters was investigated as a function of time. Particles having water contact angles of (approximate)30° (lower hydrophobic sample) and (approximate)82° (higher hydrophobic sample) were prepared and used in the aggregation experiments. In the early stage of aggregation the kinetics was found to be of the second order. The experiments revealed that the increasing particle hydrophobicity increased the rate constants in every case, which could be attributed to the increasing particle–particle attractions and the decreasing hydrodynamic resistance of particles (clusters) to motion. Moreover, the lower hydrophobicity of particles manifested itself in a more important polydispersity of clusters and an unexpected cross-over during the growth. The cluster–cluster aggregation was succeeded by a particle- (large) cluster aggregation after the first (initial) part of the growth. An off-lattice computer simulation of cluster-cluster aggregation, based on molecular dynamics, was also developed for the better understanding of the interfacial aggregation. The simulations supported well the conclusions derived from the real experiments, and gave an indispensable possibility for the study of the effect of single parameters on the complex phenomenon. © 2001 American Institute of Physics.

Notes: Polymerization of rigid rodlike molecules with reactive end groups requires near parallel orientation of the molecules. The reaction is diffusion limited because of the low mobility of the molecules in the later stages of the reaction. Experimental studies have shown that flow-induced molecular orientation enhances the rate of polymerization [Agarwal and Khakhar, Nature 360, 53 (1992)]. Here a theoretical study of the polymerization under axisymmetric extensional flow is carried out to obtain the effective reaction rate constant (keff) for the reaction. Computations show that an increase in the intrinsic rate constant (kh) results in a decrease in the relative rate constant krel=keff/kh. Reduction in the rotational diffusivity (Dr) results in a significant reduction in krel; however, the variation of the translational diffusion coefficient perpendicular to the rod axis D⊥ has only a small effect for D⊥/D||(very-much-less-than)1, where D|| is the diffusivity parallel to the rod axis. The imposition of flow increases the effective rate constant, however, the variation of krel with other parameters remains qualitatively similar at different Peclet numbers (Pe=cursive-epsilon/Dr, where cursive-epsilon is the extensional rate). To simulate the variation of the rate constant during polymerization, computations are carried out for different rod lengths using correlations to estimate rod diffusivities. Results indicate that krel initially decreases and then increases after a certain critical degree of polymerization, which reduces with increase in Peclet number. For sufficiently high extensional rates (cursive-epsilon∼200 s−1) the rate constant becomes higher than the intrinsic value (krel〉1). © 2001 American Institute of Physics.

Notes: IR absorption lines of CO on ultrathin epitaxial iron films are both enhanced and asymmetric. In this letter we show new experimental results which demonstrate a correlation of the asymmetry of the CO-stretching line to electronic properties of the underlying metal film. The new finding indicates the important role of metal film morphology for nonadiabatic effects. Such effects are strongest slightly above the percolation threshold as our results show. © 2000 American Institute of Physics.

Notes: Glasses with a composition xLi2O⋅(1−x)B2O3 were investigated by low-frequency Raman scattering in the composition range x=0–0.28. The evolution of the quasielastic line, the boson peak, the Debye frequency, and some other glass parameters with the composition was analyzed. The frequency of the boson peak ωb shifts with changing x by a factor of 3 and the width of the quasielastic spectrum at room temperature is always equal to ∼0.24ωb. The Grüneisen parameter of the glasses is estimated on the basis of the light scattering data for the boson peak frequency within the frames of the anharmonic model of the fast relaxation and using the sound velocity data—for the Debye frequency. The anharmonic properties are compared with the fragility of these glassformers; it is shown that the fragility increases with anharmonicity. It is shown also that the width of the glass transition region correlates with the anharmonic properties. © 2000 American Institute of Physics.

Notes: Electron transfer reactions at semiconductor/liquid interfaces are studied using the Fermi Golden rule and a free electron model for the semiconductor and the redox molecule. Bardeen's method is adapted to calculate the coupling matrix element between the molecular and semiconductor electronic states where the effective electron mass in the semiconductor need not equal the actual electron mass. The calculated maximum electron transfer rate constants are compared with the experimental results as well as with the theoretical results obtained in Part I using tight-binding calculations. The results, which are analytic for an s-electron in the redox agent and reduced to a quadrature for pz- and dz2-electrons, add to the insight of the earlier calculations. © 2000 American Institute of Physics.

Notes: Measurements of initial adsorption probabilities, S0, as well as the coverage dependence of the adsorption probability, S(aitch-thetaCO), of CO on Zn–ZnO [ZnO(0001)] and O–ZnO [ZnO(0001¯)] are presented. The samples have been characterized by He atom scattering, He atom reflectivity measurements, LEED, and XPS. Samples with different densities of defects were examined, either by investigating different samples with identical surface termination (for O–ZnO) or by inducing defects by ion sputtering at low temperatures (for Zn–ZnO). The influence of kinetic energy and impact angle (for Zn–ZnO) as well as adsorption temperature on the adsorption dynamics have been studied. For both polar surfaces the shape of the coverage dependent adsorption probability curves are consistent with a precursor mediated adsorption mechanism. Adsorbate assisted adsorption dominates the adsorption dynamics for high impact energies and low adsorption temperatures, especially for Zn–ZnO. The He atom reflectivity measurements point to the influence of an intrinsic precursor state. In contrast to the Zn–ZnO surface, for O–ZnO a weak thermal activation of the CO adsorption was observed. Total energy scaling is obeyed for Zn–ZnO. The heat of adsorption for CO on both polar faces varies between 7 kcal/mol (low coverage) and 5 kcal/mol (high coverage). A comparison of He atom reflectivity with S(aitch-thetaCO) curves demonstrates that CO initially populates defect sites on both surfaces. For O–ZnO an increase in S0 with decreasing density of defects was observed, whereas for the Zn-terminated surface S0 was independent of the defect density within the range of parameters studied. © 2000 American Institute of Physics.

Notes: The coadsorption of CO and butane on a Pt(533) stepped surface has been investigated using reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). The adsorption of butane on Pt(533) with CO preadsorbed on step-atop sites reveals that butane can force CO to tilt with a minimum angle of 42° away from the surface normal and displace CO from step-atop to step-bridge sites. The energy required for this tilting should be less than 20.5 kJ/mol. The coverage at which the compressed butane phase occurred was found to be the same at which this phase occurs on bare Pt(533). Together with the observed tilting and displacement of CO, this suggests that at low coverages butane adsorbs on the terraces, rotated 60° away from the step edge. The second monolayer phase then consists of tilted butane molecules having two hydrogen atoms in direct contact with the surface, situated near the step edge. The presence of butane also results in a downward shift of the CO stretch frequency, caused by electron donation in the 2π* antibonding CO orbital. When butane is preadsorbed at a submonolayer coverage exposure to CO leads to displacement of butane into a compressed phase and even into a multilayer phase. This effect becomes smaller as the initial butane coverage is increased to the multilayer regime. © 2000 American Institute of Physics.

Notes: The atomic structure and diffusion at the solid–liquid heterophase interface are investigated by using Molecular Dynamics. The system studied is made of crystalline copper with surface terminations (100) and (111) and liquid aluminum, both modeled via adapted n-body potentials from the literature and cross interactions obtained by fitting the mixing enthalpy of the two species to experimental values. It is shown that at the interface the liquid forms layers with spacing such that the local average density equals that of the bulk liquid. The interfacial liquid is layered whatever the surface orientation is even if the solid is reduced to a single crystalline or amorphous layer, in agreement with density functional theory. Layering is however suppressed at the interface between the liquid and a bulk amorphous solid with a rough surface termination. Surprisingly, diffusion in the interfacial layers proceeds via vacancies, which also accommodate the density misfit between solid (Cu) and liquid (Al). These results are further discussed in the frame of existing experimental and theoretical works. © 2000 American Institute of Physics.

Notes: Penning ionization electron spectroscopy was applied to ultrathin pentacene films [monolayer (0.3 nm thick) to dozens of layers] prepared by vapor deposition under different conditions. Remarkable differences were found among the Penning ionization electron spectra (PIES). The local electron distribution of each molecular orbital (MO) protruding from the film surface was probed and the relation between the MO shape and the molecular orientation was investigated. Deposition onto a metal substrate without a crystallographical surface yields a crystalline film at room temperature. The molecules are oriented with the long axes almost perpendicular to the substrate and make the σ bands of the PIES by far stronger than the π bands. In the pure π region, the π9 and π7 MOs having large distribution at the long-axis end provide more intense bands than other π MOs. On the metal substrate held at 213 K, molecules form an amorphous film with the long axes inclined a little on average. The π and σ bands exhibit comparable intensities and no specific band is enhanced. When 1 monolayer equivalence (MLE) of pentacene is deposited onto a graphite substrate at 123 K, a monolayer of flat-lying molecules is obtained. The π MOs provide more enhanced bands than the σ MOs but the π9 and π7 MOs with little distribution around the C–H bonds are harder to detect than other MOs in the pure π region. Furthermore, the growth of each film was investigated using Penning spectroscopy and ultraviolet photoelectron spectroscopy in combination. Spectral dependence upon amount of deposition revealed three modes of film growth, which correspond to the three molecular aggregations. The crystalline "film" cannot cover the substrate to ca. 30 MLE because molecules landed on the substrate move around and gather to form crystallites which grow three-dimensionally. But, the crystallite formation is inhibited on the cooled metal substrate owing to the low mobility of molecules. The rough surface is completely covered at 3–5 MLE and the molecules are accumulated randomly but uniformly in thickness with further deposition. On the graphite substrate, every new monolayer of flat-lying molecules is formed at 123 K and piled up in succession to form a layered film. With increasing number of layers, however, the surface molecules become inclined little by little. Finally, at 60 MLE they are tilted to the same extent as in an amorphous film. The structures and growth modes were found consistent with the stability or sublimation properties of these and related films as well as with the relaxation shifts reflected in the positions of the first PIES bands. It was also indicated that the aggregation of the outermost molecules is considerably different between the amorphous and layered film of 60 MLE in spite of similar, somewhat-tilted orientation. That is, the molecules mutually overlay and sterically prevent the neighbors from desorbing in the former, whereas the molecules lack upper-side neighbors and are very liable to desorb in the latter. © 2000 American Institute of Physics.

Notes: The structure of water around methane during hydrate crystallization from aqueous solutions of methane is studied using neutron diffraction with isotopic substitution over the temperature range 18 °C to 4 °C, and at two pressures, 14.5 and 3.4 MPa. The carbon–oxygen pair correlation functions, derived from empirical potential structure refinement of the data, indicate that the hydration sphere around methane in the liquid changes dramatically only once hydrate has formed, with the water shell around methane being about 1 Å larger in diameter in the crystal than in the liquid. The methane coordination number in the liquid is around 16±1 water molecules during hydrate formation, which is significantly smaller than the value of 21±1 water molecules found for the case when hydrate is fully formed. Once hydrate starts to form, the hydration shell around methane becomes marginally less ordered compared to that in the solution above the hydrate formation temperature. This suggests that the hydration cage around methane in the liquid may be different from that when hydrate is forming and from that found in the hydrate crystal structure. Methane–methane radial distribution functions show that methane molecules can adopt a range of separations during hydrate formation, corresponding to the more distorted nature of the methane–water correlations. There is noticeable ordering of the methane molecules with a monolayer of water molecules between them once hydrate has formed. The dipole moments of the hydrating water molecules lie mostly tangential to the methane–water axis, both before, during, and after hydrate formation. © 2000 American Institute of Physics.

Notes: The A˜ 1B1–X˜ 1A1 electronic transition of germylene has been reinvestigated. A room temperature absorption spectrum of the central portion of the 000 band of GeH2 has been obtained using the technique of laser optogalvanic spectroscopy. A rotationally resolved spectrum of the 000 band of jet-cooled GeD2 has been recorded with a pulsed discharge source. Analysis of these spectra has yielded ground and excited state rotational constants for the 74GeH2, 72GeH2, 70GeH2, 76GeD2, 74GeD2, 72GeD2, and 70GeD2 isotopomers and approximate equilibrium structures of: r″(Ge–H)=1.5883(9) Å, θ″(H–Ge–H)=91.22(4)°, r′(Ge–H)=1.5471(6) Å, and θ′(H–Ge–H)=123.44(2)°. The ground state ν1 and ν2 vibrational frequencies have been determined from wavelength-resolved fluorescence spectra of jet-cooled GeH2 and GeD2. There is good evidence that GeH2 rotational levels with Ka′〉1 are so strongly predissociated that lifetime broadening makes them diffuse, severely restricting the information that can be obtained from absorption and laser-induced fluorescence experiments. © 2000 American Institute of Physics.

Notes: We report an accurate ab initio study of the effects of chirality on the intermolecular interactions between two small chiral molecules bound by a single hydrogen bond. The methods used are second-order Møller–Plesset theory (MP2), as well as density functional theory with the B3LYP functional. The differential interaction energy between two homochiral molecules, e.g., R⋅⋅⋅R′ and the analogous heterochiral molecules R⋅⋅⋅S′ measures the degree of chiral discrimination, termed the chirodiastaltic energy, ΔEchir. Formation of the O–H⋅⋅⋅O hydrogen bond between the chiral H-bond donor HOOH and the chiral H acceptor 2-methyl oxirane leads to four diastereomeric complexes. There are two distinct contributions to the chirodiastaltic energies, the diastereofacial contribution which controls the face or side of the acceptor to which the H bond is formed, and the diastereomeric contribution, which is the energy difference between two complexes formed by (M)- and (P)-HOOH to the same face. The largest chirodiastaltic energy is ΔEchir=0.46 kcal/mol (6% of the binding energy) between the syn-(M)- and syn-(P)-HOOH⋅2-methyl oxirane complexes. The chiral 2,3-dimethyloxirane acceptor is C2 symmetric and hence offers two identical faces. Here the chirodiastaltic energy is identical to the diastereomeric energy, and is calculated to be ΔEchir=0.36 kcal/mol or 4.5% of the binding energy. © 2000 American Institute of Physics.

Notes: The A2Πu–X2Πg electronic system of gaseous C7− is examined experimentally in the light of theoretical predictions. Ab initio calculations at the RHF, RCCSD(T) and MRCI levels using the aug-cc-pVQZ basis set indicate that the transition is accompanied by a small elongation in the molecule and a significant reduction in the spin-orbit coupling constant. On the basis of these predictions the band profiles of the 000, 101, 201 and 301 transitions were recorded using photodetachment spectroscopy. These spectra revealed the spin-orbit component bands for each transition as well as providing band contours which show partially resolved rotational structure. The experimental spectra are compared to simulations based upon the calculated spectroscopic constants and the possible causes of the main features in the band contours are accessed by least-squares fitting of the profiles for the 000 and 101 transitions. The implications for the recent observation of coincidences between the A2Πu–X2Πg vibronic bands of C7− and the diffuse interstellar bands are discussed. © 2000 American Institute of Physics.

Notes: We present comprehensive 3D lattice Monte Carlo simulations of the folding kinetics of two-turn antiparallel β sheets. The model employed takes into account isotropic nonspecific interactions as in previous flexible heteropolymer models and also orientation-dependent monomer–monomer interactions, mimicking the formation of hydrogen bonds and chain rigidity. The chain length is varied from N=15 to 33. For each chain length, we calculate the fastest folding temperature, Tfast, folding temperature, Tfold, and glass-transition temperature, Tg. The time-averaged occupation probability of the native state is found to be nearly independent of N at all temperatures. The dependence of Tfast and Tfold on N is accordingly relatively weak. The temperature interval where the folding is fast rapidly decreases with increasing N. For the chain lengths chosen, Tfold slightly exceeds Tg. The dependence of the folding time τf on N is well fitted by using the power law, τf∝Nλ. The exponent λ is found to depend on temperature and on the distribution of nonspecific interactions in the chain. In particular, λ=2.7–4.0 at T=Tfast and 5.2 at T slightly below Tfold. Evaluating τf in real units at T near Tfold yields physically reasonable results. © 2000 American Institute of Physics.

Notes: © 2000 American Institute of Physics.

Notes: We report the direct ab initio calculation of vibrational energies of the chloride anion–water complex by interfacing the code MULTIMODE, which does variational calculations of vibrational energies, with GAUSSIAN, which does ab initio calculations of electronic energies. Convergence of the results with respect to the level of mode-coupling considered indicates that the present results are reliable enough to distinguish between two sets of conflicting experimental reports of these vibrational energies. © 2000 American Institute of Physics.

Notes: Applying solvation dynamics experiments to viscous liquids or glassy materials near their glass transition involves long lived triplet probes, whose time dependent phosphorescence signals depend upon the local dipolar orientational dynamics, mechanical responses, and polarities. The current understanding of experimental results regarding steady state and time dependent optical line shapes and positions is reviewed with emphasis on the relation to the macroscopic dielectric properties. Several applications are discussed in detail, where advantage is taken of the spatially local instead of ensemble averaging character of this technique. These examples include studies of dynamical heterogeneity, rotational solute/solvent coupling, secondary relaxations in the glassy state, as well as confinement and interfacial effects. © 2000 American Institute of Physics.

Notes: Ab initio calculations at the levels of Hartree–Fock (HF), second order Møller–Plesset perturbation theory (MP2), coupled-cluster theory with singles, doubles, and estimated triple excitations [CCSD(T)], and density functional theory (DFT) using the functionals B3LYP and B3PW91 of the relative energies of the C2H2S2 isomers 1,2-dithiete (2a), and dithioglyoxal (2b) show a peculiar dependence of the results on the f-type polarization functions. The ab initio calculations with 6-31G(nd) basis sets with n=1–3 incorrectly predict that 2a is higher in energy than 2b. The relative energies at the MP2 and CCSD(T) levels change by more than 6 kcal/mol in favor of 2a if the basis set is augmented by one set of f functions. The DFT calculations also give a higher stability of 2a relative to 2b if f functions are included in the basis sets, but the change in the relative energy is only ∼2 kcal/mol. The large change in the relative energies which are calculated at MP2 and CCSD(T) are mainly due to the functions at sulfur, while the effect of the f functions in the DFT calculations is mainly due to the f functions at carbon. © 2000 American Institute of Physics.

Notes: A detailed analysis that benefits from a slate of new approximate numerical and exact asymptotic results produces highly accurate properties of the ground state of the harmonium atom as functions of the confinement strength ω and quantifies the domains of the weakly and strongly correlated regimes in this system. The former regime, which encompasses the values of ω greater than ωcrit(approximate)4.011 624×10−2, is characterized by the one-electron density ρ(ω;r1) with a global maximum at r1=0. In contrast, the harmonium atom within the latter regime, which corresponds to ω〈ωcrit, differs fundamentally from both its weakly correlated counterpart and Coulombic systems. Resembling a Wigner crystal of a homogeneous electron gas, it possesses a radially localized pair of angularly correlated electrons that gives rise to ρ(ω;r1) with a "fat attractor" composed of a cage critical point and a (1, −1) critical sphere. Allowing for a continuous variation in ω, the new compact representation of the ground-state wave function and accurate approximants for the corresponding electronic properties are designed to facilitate the use of harmonium in research on electron correlation and density functionals. © 2000 American Institute of Physics.

Notes: A unified framework is developed to describe extrapolations from simulations performed in either a single system box or two subsystems at equilibrium (i.e., Gibbs ensembles). It is shown that the Gibbs ensemble can be used in conjunction with histogram reweighting and pseudo-ensemble techniques in order to map out more effectively different kinds of phase diagrams, in particular for binary and ternary systems. These extrapolation schemes allow the use of different phase-equilibrium specifications, some of which could not be simulated by conventional approaches. Novel semi-open and osmotic Gibbs ensembles are also described as counterparts of single-phase open and osmotic ensembles, respectively. Applications of the proposed methods are presented to the simulation of pressure-composition diagrams, bubble lines, and isoenthalpic partitioning. © 2000 American Institute of Physics.

Notes: The reaction of ground state carbon atoms, C(3Pj), with dimethylacetylene, H3CCCCH3, was studied at three collision energies between 21.2 and 36.9 kJmol−1 employing the crossed molecular beam approach. Our experiments were combined with ab initio and RRKM calculations. It is found that the reaction is barrierless via a loose, early transition state located at the centrifugal barrier following indirect scattering dynamics through a complex. C(3Pj) attacks the π system of the dimethylacetylene molecule to form a dimethylcyclopropenylidene intermediate either in one step via an addition to C1 and C2 of the acetylenic bond or through an addition to only one carbon atom to give a short-lived cis/trans dimethylpropenediylidene intermediates followed by ring closure. The cyclic intermediate ring opens to a linear dimethylpropargylene radical which rotates almost parallel to the total angular momentum vector J. This complex fragments to atomic hydrogen and a linear 1-methylbutatrienyl radical, H2CCCCCH3(X2A″), via a tight exit transition state located about 18 kJmol−1 above the separated products. The experimentally determined exothermicity of 190±25 kJmol−1 is in strong agreement with our calculated data of 180±10 kJmol−1. The explicit verification of the carbon versus hydrogen exchange pathway together with the first identification of the H2CCCCCH3 radical represents a third pathway to form chain C5H5 radicals in the reactions of C(3Pj) with C4H6 isomers under single collision conditions. Previous experiments of atomic carbon with the 1,3-butadiene isomer verified the formation of 1- and 3-vinylpropargyl radicals, HCCCHC2H3(X2A″), and H2CCCC2H3(X2A″), respectively. In high-density environments such as combustion flames and circumstellar envelopes of carbon stars, these linear isomers can undergo collision-induced ring closure(s) and/or H atom migration(s) which can lead to the cyclopentadienyl radical. The latter is thought to be a crucial reactive intermediate in soot formation and possibly in the production of polycyclic aromatic hydrocarbon molecules in outflow of carbon stars. Likewise, a H atom catalyzed isomerization can interconvert the 3-vinylpropargyl and the 1-methylbutatrienyl radical. © 2000 American Institute of Physics.

Notes: The atomic and molecular hydrogen elimination processes from ethylene have been studied using a molecular beam apparatus. Site and isotope effects on the molecular hydrogen elimination from ethylene have been clearly observed from the photodissociation of ethylene at 157 nm. Experimental results show that there are three different types of molecular elimination processes: 1,1 elimination, 1,2-cis elimination, and 1,2-trans elimination. Significant differences have been detected between 1,1 elimination and 1,2 eliminations in their kinetic energy distributions. Noticeable difference is also found between 1,2-cis elimination and 1,2-trans elimination for molecular deuterium elimination. Branching ratios for atomic and molecular hydrogen elimination processes have also been determined for ethylene and its isotopomers. Isotope and site effects on the branching ratios of different molecular elimination channels have been observed. The experimental results are also compared with recent theoretical studies. © 2000 American Institute of Physics.

Notes: Dispersed emission spectra collected upon the 4 2P3/2,1/2←4 2S1/2 optical excitation of K atoms attached to helium nanodroplets include broad, structured, red-shifted features which are shown to be due to K*He exciplex formation, paralleling our former observation of Na*He [J. Reho, C. Callegari, J. Higgins, W. E. Ernst, K. K. Lehmann, and G. Scoles, Discuss. Faraday Soc. 108, 161 (1997)]. The exciplex formation is demonstrated by the agreement obtained in comparing the K*He A(1) 2Π→X(1) 2Σ emission spectra with the predictions derived from available ab initio potential energy surfaces. Recent analysis of both exciplex emissions also points to the possibility of triatomic (Na*He2 and K*He2) exciplex formation for a small fraction of the alkali atoms. The lack of fluorescence quenching, which is present when the spectra are taken in bulk liquid helium, is due to the surface location of the alkali atoms on the helium droplets that allows the nascent Na*He and K*He exciplexes to desorb from the droplet and emit as isolated molecules. © 2000 American Institute of Physics.

Notes: We have monitored the time evolution of the fluorescence of K*He exciplexes formed on the surface of helium nanodroplets using reversed time-correlated single photon counting. In modeling the present data and that from our previous work on Na*He, we find that partial spin–orbit coupling as well as the extraction energy of helium atoms from the droplet contribute to the observed dynamics of both K*He and Na*He formation, which differ considerably after either D1(n 2P1/2←n 2S1/2) or D2(n 2P3/2←n 2S1/2) excitation for both K(n=4) and Na(n=3). Our quantitative prediction of the Na*He formation dynamics coupled with preliminary data on and modeling of the formation dynamics of K*He allow for extrapolation to the case of Rb*He. Spin–orbit considerations combined with a simple model of helium atom extraction from the matrix reveal the following predicted trend: as the choice of the alkali guest atom is moved down the periodic table, alkali atom–He exciplex formation along the 1 2Π3/2 surface occurs faster while formation along the 1 2Π1/2 surface occurs more slowly, ceasing to occur at all in the case of Rb. © 2000 American Institute of Physics.

Notes: We have studied a dimensional crossover of a diffusion-limited reaction A+B→0, with and without a drift in a quasi-one-dimensional lattice W×L where the length of the lattice L is large and W is the width of the lattice. The density follows a scaling function such as C(t)∼W−xf(t/tc), where f(z)∼z−α,z(very-much-less-than)1 with α=0.59(1) regardless of the drift and f(z)∼z−β,z(very-much-greater-than)1 with β=0.254(8) without the drift and β=0.31(2) with the drift. We found the scaling exponent x=0.87(1) for the isotropic diffusion and x=1.05(1) for the maximum drift. We observed that the crossover time had a power law like tc∼Wy with y=1/2(β−α). © 2000 American Institute of Physics.

Notes: We present an implicitly parallel, direct reduced list method for integral-driven full CI (FCI) algorithms to be used in CAS-SCF. Our algorithm makes efficient use of modern supercomputer hardware supporting both shared memory and distributed memory architectures. The applicability and efficiency is demonstrated with a CAS-SCF(14,14) calculation on stilbene and a CAS-SCF(8,16) calculation on pentalene. © 2000 American Institute of Physics.

Notes: Geometries, vibrational frequencies, and interaction energies of the CNH(centered ellipsis)O3 and HCCH(centered ellipsis)O3 complexes are calculated in a counterpoise-corrected (CP-corrected) potential-energy surface (PES) that corrects for the basis set superposition error (BSSE). Ab initio calculations are performed at the Hartree–Fock (HF) and second-order Møller–Plesset (MP2) levels, using the 6-31G(d,p) and D95++(d,p) basis sets. Interaction energies are presented including corrections for zero-point vibrational energy (ZPVE) and thermal correction to enthalpy at 298 K. The CP-corrected and conventional PES are compared; the uncorrected PES obtained using the larger basis set including diffuse functions exhibits a double well shape, whereas use of the 6-31G(d,p) basis set leads to a flat single-well profile. The CP-corrected PES has always a multiple-well shape. In particular, it is shown that the CP-corrected PES using the smaller basis set is qualitatively analogous to that obtained with the larger basis sets, so the CP method becomes useful to correctly describe large systems, where the use of small basis sets may be necessary. © 2000 American Institute of Physics.

Notes: In this work we compare the generalized Fulton–Gouterman transformation (GFGT) and the Lee–Low–Pines transformation (LLPT). We apply both to a general electron-phonon Hamiltonian including the Fröhlich Hamiltonian, an electronic periodic potential, phononic anharmonicities, and Umklapp processes. We respectively discuss the ability of the two unitary transformations to achieve approximate electronic diagonalization in general and also for a particular electronic Bloch base. We further consider their possible application to multiparticle systems. It turns out that in general the GFGT is a more successful technique than the LLPT. © 2000 American Institute of Physics.

Notes: The dependence of the total electron energy density at the (3,−1) critical point (CP) of the H...O interaction against the interatomic distance (ECP) has been obtained by the addition of the local electron kinetic (GCP) and potential (VCP) energy densities dependences (ECP=GCP+VCP) for a set of 83 X-H...O hydrogen bonds (X=C, N, O). The ECP function has been related to the interaction potential by means of a proportionality relationship U=−υ⋅ECP, υ being a positive constant in volume units. Based on the GCP and VCP functionalities, the proposed H...O interaction potential has been successfully checked against several physical and chemical properties. The behavior of the U function has been compared to Morse and Buckingham-type potentials, leading to an almost perfect matching between all of them when they were constrained to have the same three parameters: the potential well depth U0, its position r0, and the curvature of the potential function at r0. The resulting U(r) function is simply described by the addition of two exponential terms: U(r)=49 100 exp(−3.6r)−11 800 exp(−2.73r), where U is in kJ/mol and r is the H...O distance in Å. © 2000 American Institute of Physics.

Notes: Variational calculations of excited vibrational states for the OCS molecule, using generalized internal coordinates properly optimized, are presented. The calculations are made for two empirical and one ab initio potential energy surfaces previously reported. It is shown that the computed vibrational frequencies differ considerably from the experimental values for the three potential surfaces employed. Consequently a new and much more accurate potential surface is determined for OCS by nonlinear least-squares fitting to the observed vibrational terms. The surface is expressed as a Morse-cosine expansion in valence coordinates and its quality is checked by computing the vibrational frequencies of three isotopic species of the molecule. © 2000 American Institute of Physics.

Notes: Grand canonical molecular dynamics simulations have been performed to determine the sizes of critical nuclei in gas–liquid nucleation. The studied system consists of Lennard-Jones (LJ) argon atoms with a potential cutoff of 4.9σ at a reduced temperature of 0.694 and at vapor supersaturations between 3.5 and 6.2. To facilitate comparison with nucleation theories, we have also determined the equilibrium vapor pressure of LJ argon as a function of temperature. The results are compared with other simulation studies, and with predictions of the classical (CNT) and density functional (DFT) nucleation theories. We find that the semiempirical version of the DFT is in excellent agreement with the simulation results, and that the CNT underestimates the number of LJ atoms in the nuclei only very slightly. © 2000 American Institute of Physics.

Notes: Small-angle x-ray and neutron scattering are widely used techniques to study the structure of colloidal particles in the size range up to 100 nm. The indirect Fourier transformation technique is well established to obtain model free real space information, but the interpretation of the results is limited to cases where particle interaction can be neglected. The extended generalized indirect Fourier transform (GIFT) allows one to separate inter- and intraparticle effects, but needs models for the particle interaction. We present the application of three different models for the calculation of interaction effects of charged particles, represented by the structure factor. With this extension, useful real space information can be obtained by the GIFT method for solutions with volume fractions up to about 0.3 without any assumption for the shape of the particles. Only the interaction effects need a model assumption, and the parameters determined from this model can give some additional information. Simulations show that it is impossible to determine charge and ionic strength simultaneously. There exists another ambiguity between the parameter sets for charge, radius, and volume fraction, but we show how this problem can be overcome in most cases. The practical applicability of the method is demonstrated by means of the micellar system CTAB in different concentrations from 1% up to 20% and with varying amounts of added salt to screen the charges and change the particle shape. © 2000 American Institute of Physics.

Notes: Using time domain reflectometry, we carried out dielectric relaxation measurements on 1-propanol–water mixtures for the entire concentration range in the frequency range 100 MHz–25 GHz at 20, 25, and 30 °C. We have calculated the excess partial molar activation free energy, enthalpy, and entropy for 1-propanol, ΔG1PAE, ΔH1PAE, and ΔS1PAE, and those for water, ΔGWE, ΔHWE, and ΔSWE from the relaxation times. In the region of X (molar fraction of 1-propanol) ≥0.14, ΔH1PAE and ΔS1PAE take nearly zero. This means that 1-propanol molecules in the mixtures find themselves in not a very different environment from that in pure liquid. In the water-rich region, ΔH1PAE and ΔS1PAE exhibit two maxima at X∼0.03 and X∼0.06, corresponding roughly to 0.9 and 0.78 g/cm3 of water content, respectively. This fact, together with the results of the molecular dynamics studies of Sciortino et al. suggest the formation of two kinds of saturated hydration structures: the clathrate hydration shells with tetrahedral local arrangements of water molecules around X∼0.03 and nonclathrate shells with large cavities with three-coordinated local arrangements around X∼0.06. Hydrophobic hydration seems to partly share the same mechanism with structural enhancement in pure water by lowering local density. © 2000 American Institute of Physics.

Notes: We have devised a novel importance sampling method for nonequilibrium processes. Like transition path sampling, the method employs a Monte Carlo procedure to confine or bias the search through trajectory space. In this way, molecular dynamics trajectories consistent with the nonequilibrium dynamics of interest are generated efficiently. Using results of this sampling, we demonstrate that statistics of the energy gap between a solute's electronic states are Gaussian throughout the dynamics of nonequilibrium solvation in water. However, these statistics do change in time, reflecting linear response that is nonstationary. Discrepancies observed between the dynamics of nonequilibrium relaxation and of equilibrium fluctuations are thus explained. We analyze a simple Gaussian field theory that accounts for this nonstationary response. © 2000 American Institute of Physics.

Notes: The partition function, Q, of H2〈sup ARRANGE="STAGGER"〉16O is calculated by explicit summation of about 10 500 experimental vibration-rotation energy levels and very high accuracy estimates are obtained for the specific heat capacity (Cp), the Gibbs enthalpy function (gef), the Helmholtz function (hcf) and the entropy (S) of gas phase water as a function of temperature. For temperatures above 600 K it is necessary to augment the sum with theoretical estimates of the energy levels. These are obtained from high accuracy variational calculations which are extended to dissociation using a model for rotational levels based on a Padé approximant. Estimates for the partition function and other thermodynamic quantities are obtained for temperatures up to 6000 K and temperature dependent error bars presented. All estimates are highly accurate with the exception of Cp for T〉5000 K, for which further work is required. © 2000 American Institute of Physics.

Notes: We have performed Monte Carlo simulations of a three-dimensional quenched-annealed system on a cubic lattice with nearest-neighbor interactions. A small fraction of the lattices sites are blocked, thereby creating a quenched matrix. Histogram reweighting techniques are applied to investigate the critical behavior of the system. We have studied lattice sizes ranging from L=10 to L=18. For each size, we have evaluated the number of matrix replicas necessary to obtain statistically meaningful results. This number, determined by analyzing the convergence of the histograms, ranged from 50 for the smallest system sizes to 200 for the largest sizes. We have evaluated the critical temperature, the fourth cumulant of Binder et al. [K. K. Kaski, K. Binder, and J. D. Gunton, Phys. Rev. B 29, 3996 (1984)], and the critical exponents 1/ν and β/ν. The estimated critical temperature is only slightly lower than that of the three-dimensional Ising model. The simulated critical exponents, however, differ significantly from those for Ising-class three- and two-dimensional systems. © 2000 American Institute of Physics.

Notes: We have modeled the orientational dynamics of benzene in Na–Y zeolite, motivated by the NMR study of Isfort et al. at loadings of five benzenes per cage [Chem. Phys. Lett. 288, 71 (1998)]. We consider guest-guest interactions in two stages: first, we include only site blocking; next, we consider both site blocking and nearest-neighbor attractions. We calculated orientational correlation functions using kinetic Monte Carlo and also with a mean field master equation (MFME). Both methods produce correlation functions exhibiting biexponential decay in time. Analytically solving the MFME shows that long-time decay is controlled by a composite of intracage and cage-to-cage jumps. The apparent activation energy is greater than the fundamental cage-to-cage barrier when considering only site blocking, but is less than the same fundamental barrier when also including guest-guest attractions. This suggests that the actual cage-to-cage barrier is greater than the 40 kJ mol−1 reported by Isfort et al., which lends credence to previous simulations of benzene in Na–Y. © 2000 American Institute of Physics.

Notes: We have described a method that maximizes the phase separation of graphitic particles (GP) and multiwalled carbon nanotubes (MWNT) in solutions of various organic polymeric hosts. This involves the formation of sediment and a solute. These components were characterized for MWNT and GP content using electron paramagnetic resonance (EPR) measurements. All EPR signals could be deconvoluted into nanotube and GP components. When normalized, these components are representative of the mass of MWNT and GP present. This allows us to make quantitative measurements of nanotube and GP content in different environments. The most successful polymer host was poly (m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PmPV). In this case the solute contained 63% of the added nanotubes with only 2% of the added graphite remaining. © 2000 American Institute of Physics.

Notes: Melting of crystalline solids (superlattices) of octadecanethiol and octanethiol protected silver clusters has been studied with x-ray powder diffraction (XRD), differential scanning calorimetry (DSC), and infrared (IR) spectroscopy. These solids have been compared with the silver thiolate layered compounds in view of their similarity in alkyl chain packing and x-ray diffraction patterns. Superlattice melting is manifested in XRD around 400 K as the complete disappearance of all the low angle reflections; only bulk silver reflections due to the cluster cores are seen at 423 K. The superlattice structure is regained upon cooling from a temperature close to its melting point. However, cooling from a higher temperature of 473 K does not regain the superlattice order, whereas thiolate melting is repeatedly reversible even at these temperatures. Transmission electron microscopy suggests aggregation of clusters during heating/cooling cycles. DSC shows two distinct transitions, first corresponding to alkyl chain melting and the second corresponding to superlattice melting. Only alkyl chain melting is observed in variable temperature IR and increased order is manifested upon repeated heating/cooling cycles. Alkyl chain assembly shows strong interchain coupling leading to factor group splitting in cluster superlattices upon annealing. In thiolates only one melting feature is seen in DSC and it produces gauche defects, whereas significant increase in defect structures is not seen in superlattices. Repeated heating/cooling cycles increase interchain interactions within a cluster and the superlattice order collapses. © 2000 American Institute of Physics.

Notes: The pressure profiles across a liquid–vapor interface introduced previously [J. Chem. Phys. 106, 635 (1997)] have been evaluated with the aid of molecular dynamics simulations for a system of particles interacting via a (truncated and shifted) Lennard-Jones potential. This investigation extends earlier results [J. Chem. Phys. 106, 645 (1997)] to spherical interfaces. Further evidence is found that, for the range of curvatures investigated, the surface tension is curvature independent while the investigation of larger curvatures is prevented by the considerable noise found on the liquid side of the interface. © 2000 American Institute of Physics.

Notes: Nucleation and size dependent growth of nanometer sized crystalline particles in glassy media have been studied by numerically solving the Turnbull–Fisher master equations that describe the time evolution of cluster population. Time dependencies of the formation rate and number density are determined for large clusters (built of up to 2×105 formula units, containing 1.8×106 atoms). We demonstrate that the formation rate and number density of such clusters are well approximated by Shneidman's asymptotically exact analytical solution. A quantitative test of the kinetic Turnbull–Fisher model has been performed: Evaluating the kinetic coefficients and interfacial parameters from the transient time and steady-state nucleation rates measured on six stoichiometric oxide glass compositions (lithium–disilicate, barium–disilicate, lithium–diborate, wollastonite, 1:2:3 and 2:1:3 soda–lime–silica glass compositions), we calculated the macroscopic growth rates and compared with experiments. For wollastonite, lithium–diborate and the 1:2:3 soda–lime–silica glass, differences of 2 to 4 orders of magnitude have been observed between theory and experiment. This inadequacy of the microscopic kinetic parameters in describing macroscopic growth cannot be explained by either the curvature effect on the interfacial free energy or the self-consistency correction for the cluster free energy. The origin of the discrepancy is discussed. © 2000 American Institute of Physics.

Notes: The potential energy surface (PES) search of the N-hydroxyurea dimer was searched with second-order Møller–Plesset perturbation theory (MP2) and the 6-31G(d,p) basis set. Eight local minimum energy structures have been found. Four of them have relatively strong (ΔE∼−10 to −13 kcal/mol) intermolecular interactions and the others are moderately strongly interacting species (ΔE∼−3 to −7 kcal/mol). Final estimation of interaction energies was performed using the larger 6-311G(df,pd) and 6-311G(2df,2pd) basis sets. The predicted interaction energies are ΔE=−14.26 kcal/mol and −3.43 kcal/mol for the strongest and the weakest interacting forms of the studied complex, respectively, at the MP2/6-311G(2df,2pd)//MP2/6-31G(d,p) level of theory. The self-consistent field (SCF) interaction energy decomposition indicates the important influence of the deformation term magnitude on ΔE(SCF). The calculated electron correlation contribution to ΔE(MP2) depends on the geometry of the system and varies from −0.5 to −5 kcal/mol. The estimated influence of water on the stability (free energy of hydration) of N-hydroxyurea dimers using the self-consistent isodensity polarized continuum (SCI-PCM) model of solvation varies from ∼−11 kcal/mol to ∼−21 kcal/mol. The forms predicted to be more strongly interacting species in gas phase are less influenced by hydration than the more weakly interacting ones. © 2000 American Institute of Physics.

Notes: A combined computational and experimental study of the methyl-p-aminobenzoate(H2O)n, (n=2,3,4) complexes [MAB(H2O)n] is reported. Complexes potential energy surfaces were explored by ab initio density functional theory (DFT) methods, at the B3LYP/6-31G level, and the stable isomer structures and vibrational modes further computed at the B3LYP/6-31+G* level. A set of self-contained experimental techniques, including laser induced fluorescence (LIF), resonance enhanced multiphoton ionization mass-resolved spectroscopy (REMPI), two-color resonance enhanced multiphoton ionization mass-resolved spectroscopy (R2PI), "hole burning" spectroscopy (HB), and two-color ionization thresholds were used to study the spectra and other physical features of the complexes. Of the three title complexes only MAB(H2O)4 has been observed with our experimental methods, while the MAB(H2O)3 was formed by evaporation and MAB(H2O)2 was not detected at all. It has been shown that the observed MAB(H2O)4 complex has only one isomer with a hydrogen bonded water ring structure attached to the amino hydrogens and its low vibrational modes (up to 165 cm−1) have been assigned. A discussion of the results, including structures of stable isomers, isomer energies, ionization thresholds, and the difficulties in observing some solvated complexes is presented. © 2000 American Institute of Physics.

Notes: Relativistic core-potential calculations have been carried out on Ω states resulting from the interaction of Ar*(3p54s, 3P, 1P) with ground state Ne atoms. The results yield the correct asymptotic limits for the atomic states of Ar while shallow minima (700–800 cm−1) at large internuclear distances, 7–8 bohr, are obtained for the excited states. Dipole transition moments between pairs of states have been calculated and strong radiative transitions are predicted from excited states to the ground state. The 1(I) state, correlating with the metastable 3P2 state of Ar is found to have a small dipole transition moment at short and intermediate nuclear distances leading to a radiative lifetime for this state of 8.3 μs. © 2000 American Institute of Physics.

Notes: A new method of Franck–Condon (FC) factor calculation for nonlinear polyatomics, which includes anharmonicity and Duschinsky rotation, is reported. Watson's Hamiltonian is employed in this method with multidimensional ab initio potential energy functions. The anharmonic vibrational wave functions are expressed as linear combinations of the products of harmonic oscillator functions. The Duschinsky effect, which arises from the rotation of the normal modes of the two electronic states involved in the electronic transition, is formulated in Cartesian coordinates, as was done previously in an earlier harmonic FC model. This new anharmonic FC method was applied to the simulation of the bands in the He I photoelectron (PE) spectrum of ClO2. For the first band, the harmonic FC model was shown to be inadequate but the anharmonic FC simulation gave a much-improved agreement with the observed spectrum. The experimentally derived geometry of the X˜ 1A1 state of ClO2+ was obtained, for the first time, via the iterative FC analysis procedure {R(Cl–O)=1.414±0.002 Å, (angle)O–Cl–O=121.8±0.1°}. The heavily overlapped second PE band of ClO2, corresponding to ionization to five cationic states, was simulated using the anharmonic FC code. The main vibrational features observed in the experimental spectrum were adequately accounted for in the simulated spectrum. The spectral simulation reported here supports one of the two sets of published assignments for this band, which was based on multireference configuration interaction (MRCI) calculations. In addition, with the aid of the simulated envelopes, a set of adiabatic (and vertical) ionization energies to all five cationic states involved in this PE band, more reliable than previously reported, has been derived. This led also to a reanalysis of the photoabsorption spectrum of ClO2. © 2000 American Institute of Physics.

Notes: We study the phase behavior of the Zwanzig model of suspensions of hard rods, allowing for polydispersity in the lengths of the rods. In spite of the simplified nature of the model (rods are restricted to lie along one of three orthogonal axes), the results agree qualitatively with experimental observations: the coexistence region broadens significantly as the polydispersity increases, and strong fractionation occurs, with long rods found preferentially in the nematic phase. These conclusions are obtained from an analysis of the exact phase equilibrium equations. In the second part of the paper, we consider the application of the recently developed "moment free energy method" to the polydisperse Zwanzig model. Even though the model contains nonconserved densities due to the orientational degrees of freedom, most of the exactness statements (regarding the onset of phase coexistence, spinodals, and critical points) derived previously for systems with conserved densities remain valid. The accuracy of the results from the moment free energy increases as more and more additional moments are retained in the description. We show how this increase in accuracy can be monitored without relying on knowledge of the exact results, and discuss an adaptive technique for choosing the extra moments optimally. © 2000 American Institute of Physics.

Notes: We measure the absorption spectrum of a rhodamine 101 solution in ethanol in the spectral region 555–583 nm as a function of the incident light fluence. We show that increasing the field fluence results in a blueshift of the spectrum and absorption depletion with no spectral broadening. These results are explained in terms of a model that includes the influence of the solvent over the spectrum of the dye molecule. © 2000 American Institute of Physics.

Notes: We investigate the free energy barrier of vapor tube formation in a metastable liquid confined between hydrophobic walls. The model we use is a lattice gas model with nearest neighbor interactions whose evaporation dynamics has been reported in the preceding paper (paper I). We apply transition state theory and a constrained umbrella sampling technique, taking as our transition state a vapor pocket in the middle of the liquid layer. The calculated transmission coefficients show that the size of a vapor pocket is indeed a reasonable order parameter to describe capillary evaporation. The umbrella sampling method gives estimates of free energy barrier for vapor tube formation that are within an order of magnitude agreement with direct Monte Carlo simulation runs. In all the cases studied, the estimated free energy barriers are much smaller than those predicted by a previous mean-field approach. © 2000 American Institute of Physics.

Notes: Capillary evaporation (cavitation) has been suggested to be a possible source of long range interactions between mesoscopic hydrophobic surfaces. While evaporation is predicted by thermodynamics, little is known about its kinetics. Glauber dynamics Monte Carlo simulations of a lattice gas close to liquid–gas coexistence and confined between partially drying surfaces are used to model the effect of water confinement on the dynamics of surface-induced phase transition. Specifically, we examine how kinetics of induced evaporation changes as the texture of hydrophobic surfaces is varied. Our results provide guidelines for efficient manipulation of surface properties. We find that evaporation rates can be considerably slowed upon deposition of relatively small amount of hydrophilic coverage. The distribution of hydrophilic patches is however crucial, with the regularly spaced distribution being much more effective in slowing the formation of vapor tubes that trigger the evaporation process. To relate simulation rates to experimental ones, we also perform calculations using the mass-conserving Kawasaki algorithm. We predict evaporation time scales that range from hundreds of picoseconds in the case of mesoscopic surfaces ∼104 nm2 to tens of nanoseconds for smaller surfaces ∼2×102 nm2, when the two surfaces are ∼10 solvent layers apart. The present study demonstrates that cavitation is kinetically viable in real systems and should be considered in studies of processes at confined geometry. © 2000 American Institute of Physics.

Notes: A temperature-dependent, low-frequency Raman study for a strong and a fragile glass-forming liquid is reported in order to elucidate the frequency and temperature sensitivity of the depolarization ratio spectrum, ρ(ω,T). Changes observed in ρ(ω,T) are directly reflected on the spectral features of the Raman coupling coefficient, Cαβ(ω). Our data provide evidence for polarization dependence of this coefficient, which has been completely overlooked in studies concerning the experimental determination of Cαβ(ω) through a comparison of neutron and reduced Raman spectra. The current status considering the frequency dependence of the coupling coefficient is briefly reviewed from the theoretical, experimental, and computer simulation points of view. The experimental data suggest that a reconsideration of the approaches employed should be undertaken. © 2000 American Institute of Physics.

Notes: We use the Kubo–Anderson sudden jump approach to investigate line shapes of single molecules (SMs) interacting with randomly distributed two level systems (TLSs). Depending on their random environment, SMs exhibit a wide variety of behaviors. Under certain conditions, given in the text, line shapes exhibit simple behavior, e.g., cases where lines are Lorentzian with a width which varies from one molecule to the other. As control parameters are changed a transition to complex line shape phenomena is observed (i.e., the line shapes have random structures, each with a random number of peaks). We investigate these behaviors for two cases—(i) the case when all TLSs are identical though randomly distributed in space and (ii) the standard tunneling model of low temperature glass where the TLSs are nonidentical. We show that, in certain limits, both models can be analyzed using Lévy-stable laws. For the glass model we compute the distribution of line shape variance and discuss a previous proposition, that distribution of variance and the distribution of linewidth measured in experiment are related. For the line shape problem of SMs in glass we show that background TLSs, defined in the text, can be treated collectively using a simple Gaussian approximations. The Gaussian approximation for the background reduces the number of TLSs needed for a full size simulation of the SM glass system. © 2000 American Institute of Physics.

Notes: The kinetics of multiplet chemically induced dynamic electron polarization (CIDEP) generation for radical pair (RP) recombination in low dimensional media (the dimensionality n≤3) is theoretically analyzed in detail. Simple analytical expressions for the time dependent amplitude Pe(t) of the multiplet CIDEP are derived. These expressions show that Pe(t) is sensitive to the dimensionality only in the limit of weak spin dependent interaction Q and large diffusion coefficient D, when ξ=Qd2/D(very-much-less-than)1, where d is the distance of closest approach. In the opposite limit ξ〉1, the dependence Pe(Q) is universal Pe(Q)∼Q/(1+ΛQ/D), where Λ∼d. © 2000 American Institute of Physics.

Notes: The electronic structure of layered tantalum dichalcogenides 1T-TaX2 (X=S, Se, Te) have been studied both with the linear muffin tin orbitals-atomic sphere approximation (LMTO-ASA) and the Amsterdam density functional for band (ADF-band) programs. The first code (LMTO) provides band structures, density of states (DOS), and crystal orbitals Hamiltonian populations (COHP) while the second one allows accurate atomic charge calculations by means of a powerful electron density numerical integration. All those analyses were used to rationalize the electronic structures of the three 1T-TaX2 phases, in particular to enlighten the 13×13 structural modulations observed in TaS2 and TaSe2, and to put forward the influence of the local chemical Ta–Te bonds on the relative stability of the 1T-TaTe2 phase vs the distorted monoclinical one. The indirect overlap between the two bands responsible for the metallic properties of TaS2 and TaSe2 has been shown to significantly increase the tantalum d electron count compared to its formal value (d1) leading to a more realistic occupation of the threefolded t2g-like bands involved in the 13×13 instability. Owing to the low electronegative character of Te compared to S and Se, the direct overlap occurring at the Fermi level results in an electron transfer from local Ta–Te bonding states to local Ta–Te antibonding ones yielding a destabilization of the metal–chalcogen bonds. © 2000 American Institute of Physics.

Notes: This contribution presents a systematic light scattering study of a series of boron oxide glasses which are characterized by different thermal histories. The thermal treatment was obtained by annealing the samples close to the glass transition temperature for times of several hours. Both low-frequency (0.1–30 cm−1) and high-frequency (5–1600 cm−1) spectra were monitored by using a tandem Fabry–Perot interferometer and a Raman spectrometer, respectively. The low-frequency spectra include quasielastic contributions and the boson peak. It was found that different thermal histories lead to pronounced changes in the low-frequency spectrum. The position of the boson peak shifts to higher frequencies and the magnitude of the quasielastic contribution decreases as a function of annealing time. Both quantities correlate linearly with the density of the samples (ρ=1.804–1.866 g/cm3). On the other hand, the high-frequency modes do not show discernible changes. In particular, no alteration of the modes which correspond to the boroxol ring is found, indicating that the fraction of boroxol rings is constant within 2% accuracy. Taking the boson peak as a manifestation of medium-range order, we conclude that annealing the glass influences the intermediate-range order rather than the short-range order. © 2000 American Institute of Physics.

Notes: The manner in which most molecules reorient in liquids bears little resemblance to the process in the gas phase. For small-moment-of-inertia species such as the hydrides, however, the observation of discrete spectroscopic lines corresponding to individual isolated-molecule quantum transitions suggests that one is actually seeing single-molecule dynamics perturbed only weakly by the environment—just as one sees with solution-phase vibrational behavior. We examine here the degree to which such individual rotational quantum states remain well defined in liquids by considering the rates of discrete energy-level-to-energy-level transitions in solution. For rotational quantum states that do preserve their free-rotor character in a liquid, we find that the transition rate between angular momentum states obeys a rotational Landau–Teller relation strikingly similar to the analogous expression for vibration: the rate is proportional to the liquid's rotational friction evaluated at the transition frequency. Subsequent evaluation of this friction by classical linearized instantaneous-normal-mode theory suggests that we can understand this relationship by regarding the relaxation as a kind of resonant energy transfer between the solute and the solution modes. On specializing to the particular cases of H2 and D2 in Ar(l), we find that the most critical modes are those that move the light solute's center of mass with respect to a single nearby solvent. This observation, in turn, suggests a generalization of instantaneous-normal-mode ideas that transcends both linear coupling and harmonic dynamics: an instantaneous-pair theory for the relaxation of higher-lying levels. By employing a linearized instantaneous-normal-mode theory of relaxation within the liquid band and an instantaneous-pair theory for higher-frequency relaxation, we find that the resonant-transfer paradigm is reasonably successful in reproducing molecular dynamics results spanning a wide range of different rotational states. © 2000 American Institute of Physics.

Notes: This work applies the molecular dynamics simulation method to study a Lennard-Jones liquid thin film suspended in the vapor and to explore the film thickness effect on its stability. For the accurate estimation of local pressure distributions in the film, an improved method is proposed and used. Simulation results indicate that profiles of the local surface tension distribution vary widely with film thickness, while surface tension values and density profiles show little variation. As the film gets thinner, the two liquid–vapor interfacial regions begin to overlap and liquid-phase molecules in the center region of the film experience larger tension in the direction parallel to the film surface. Such interface overlapping is believed to destabilize the film and the occurrence of film rupture depends on the system temperature and the cross-sectional area of the computational domain. © 2000 American Institute of Physics.

Notes: The melting point of aluminum has been obtained in an ab initio molecular dynamics calculation by determination of the free energies of the solid and liquid phases as a function of temperature along the zero pressure isobar. The focus of the article is to demonstrate the problems which can arise in obtaining adequately sampled free energies. The time scale on which "adiabatic switching" may be performed to calculate the free energy of the ab initio system relative to a classical reference state is discussed. To provide a consistency check, two reference states for this thermodynamic integration are used, the one component plasma and the Lennard-Jones (LJ) system. These illustrate particular difficulties which can arise. In the LJ case, for example, the intermediate fluid states which arise in integrating from the LJ fluid to the full ab initio description of Al are found to freeze. Ultimately, consistent results are obtained. © 2000 American Institute of Physics.

Notes: The interface between the [001] face of crystalline aluminum and the coexisting liquid has been studied in an ab initio molecular dynamics simulation using the orbital-free density functional description of the electronic structure. Direct observation of the equilibrium condition gives a melting temperature in excellent agreement with that obtained from the thermodynamic considerations described in the preceding paper [J. Chem. Phys. 113, 5924 (2000)]. With the resolution which can be achieved, no Friedel-type oscillations in the electron density across the interface can be seen. The atomic density profile shows two or three layers extending into the fluid. The first atomic layer beyond that at which the average atomic density falls to the bulk liquid value shows appreciable in-plane order. Monitoring the instantaneous in-plane "scattering intensity" shows that this layer fluctuates in and out of an ordered state on a time scale of picoseconds. In-plane atomic diffusion is slightly faster than interplane diffusion for these first liquid layers. © 2000 American Institute of Physics.

Notes: A newly developed expanded grand-canonical formalism is applied to locate the critical point of systems of long polymeric molecules. Two polymer systems are investigated in this work; the first consists of chains in a simple cubic lattice, the second consists of bond-fluctuating molecules. For the former we simulate molecules of up to 16 000 sites, and for the latter we study molecules of up to 500 sites. These chain lengths are well above those investigated by all prior simulation studies of critical phenomena in polymer solutions. Critical parameters are determined as a function of chain length by means of field-mixing finite-size scaling techniques. Our results for the scaling behavior of the critical temperature are consistent with literature values. Our results for the scaling of the critical density, however, indicate that the corresponding critical exponent is higher than that reported by previous authors. The leading logarithmic term of the finite-chain-length correction to the critical density is confirmed by our results. © 2000 American Institute of Physics.

Notes: The micro- and nanomorphology of surface enhanced Raman scattering (SERS) active rough substrates obtained by plasma oxidation-reduction cycles onto original flat silver surfaces have been investigated by means of a dual technique approach. Scanning force microscopy and low-frequency Raman spectroscopy give complementary results when applied on very rough systems. Almost spherical silver colloids have been used as well-defined systems to model, by their stacking over flat silicon wafers, the plasma roughening process inducing SERS activity. The SERS activity results are strongly related to the micromorphology of the nanoparticles assembly, rather than to the silver cluster size. In particular an electromagnetic enhancement factor of 103 for the breathing mode of the polystyrene aromatic rings was found to be related to the vertical stacking of tens of clusters about 10 nm in diameter. © 2000 American Institute of Physics.

Notes: Unrestricted Hartree–Fock calculations for a one-dimensional infinite periodic system have been employed to characterize a cross-talk system between trans-1,4-polybutadiene and a small molecule, O2. The total energy, the energy band structure, and the longitudinal linear polarizability have been investigated. The presence of O2 has been found to influence in a quantitatively as well as a qualitative way the energy band structure of polybutadiene. © 2000 American Institute of Physics.

Notes: Solutions of disk-shaped molecules that self-assemble into linear aggregates pose an interesting question as to whether or not it might sometimes be appropriate to regard the solute as an amphiphile and, if so, how could the strength of this amphiphilic behavior be quantified or measured. This paper proposes an answer by mapping linear self-assembly onto the isodesmic chemical equilibria of an exactly solvable one-dimensional model. In particular, the concentration dependence of the aggregation number is seen to be dominated by one of two different regimes, depending on the presence and strength of a solvophobic solute core. The amphiphilic regime is associated with a large value of the Henry Law constant for solvating the two ends of a linear aggregate. Here, the aggregation is driven by solvent "pressure" and the aggregation number rises rapidly with increasing concentration to quickly saturate at its high concentration value. In the opposite regime, the linear self-assembly is driven by solute–solute attraction and the aggregation number displays the well-known square-root-concentration form. This analysis defines protocols for identifying and classifying discotic amphiphiles, from experimental data on any aspect of a linear aggregation distribution. © 2000 American Institute of Physics.

Notes: We have investigated with extensive Monte Carlo simulations in the isothermal isobaric ensemble a system of N=8192 elongated attractive-repulsive biaxial Gay–Berne (GB) particles. We have found uniaxial and biaxial nematic phases and a biaxial orthogonal smectic phase in this thermotropic model system. © 2000 American Institute of Physics.

Notes: The salt effect on the phase transition of N-isopropylacrylamide (NIPA) gel was studied. The swelling behavior of the NIPA gel strongly depends on the salt concentration and is well described as a function of the chemical potential difference of water molecules in solution from that at the transition. From the analysis of the OH stretching, Raman spectra in water and in various aqueous solutions in terms of collective proton motions reveals that the presence of salts tends to disrupt or distort the water molecules in hydrophobic hydration shell around the NIPA gel. This leads to inducing the growth of the cluster shell around the salts, which leads to gel collapse. The volume phase transitions due to the different types of perturbation (temperature, salt) are induced by the same mechanism, hydrophobic hydration and dehydration, and therefore can be described in a unified manner in terms of the chemical potential and the collective proton motions of water molecules. © 2000 American Institute of Physics.

Notes: The vibrational coupling between the ribose and base rings of nucleic acids is modeled by the ribose–guanine vibrational interaction in ribosyl guanosine at the density functional theory (DFT) level. Two coupling patterns are revealed for the in-plane guanine vibrations depending on the strength of the kinematic interaction between ribose and guanine through the glicosidic bond. For relatively weak interactions the coupling can be described in terms of a resonance between two vibrations which are originally close in frequency (the difference in frequency between the vibrations is within ∼20 cm−1). This coupling produces two modes corresponding to the in-phase and out-of-phase combinations of the original ribose and guanine vibrations, analogous to the symmetric and antisymmetric coupled modes of the carbonyl groups in anhydrides, imides, and 1,3-diketo compounds. For strong interactions involving a significant glicosidic bond stretch, the ribose and guanine moieties can no longer be considered as quasi-independent subsystems preserving the forms of their inherent vibrations. An unambiguous identification of the original ribose and guanine vibrations contributing to these combined modes is hardly possible. Taking into account (i) the large number of intrinsic ribose and base vibrations which can potentially participate in the coupling and (ii) the resonant character of many of these interactions, these results suggest that small changes in the ribose ring conformation and glicosidic bond orientation should result in noticeable changes of the related combined modes. This explains the phenomenon of high conformational sensitivity of the corresponding conformational marker bands of nucleic acids in vibrational spectroscopy. © 2000 American Institute of Physics.

Notes: The polymerization of solid acetylene under pressure has been studied by Fourier transform infrared (FTIR) spectroscopy. Controlled laser irradiation cycles and the employment of infrared sensors to measure the sample pressure, allowed to separate the photochemical and the pressure effect on the injection and on the evolution of the reaction. The careful assignment of all the spectral features and analysis of their relative intensities and frequencies gave evidence to the specific effect of pressure and laser irradiation on the reaction products. Pressure induces an ordered growth of trans-polyenic species, while irradiation produces the opening of the double bonds and a consequent branching of the chains. Constant pressure measurements allowed to obtain precise information on the kinetics of the reaction. A monodimensional growth geometry, involving the molecules on the bc plane, agrees with the parameters extracted by the kinetic curves. Comparison between experiments at different temperatures suggests an activation of the reaction essentially due to the translational lattice modes. © 2000 American Institute of Physics.

Notes: The dynamics of three monodisperse linear duplex DNA fragments—a 2311 base pair restriction fragment and 1500 and 1100 base pair polymerase chain reaction fragments—in dilute solution are studied as functions of added salt (NaCl) concentration by dynamic light scattering-photon correlation spectroscopy. Translational diffusion coefficients and intramolecular relaxation times are extracted from the measured light scattering intensity time autocorrelation functions as the added salt concentration is reduced from 100 mM to approximately 0.1 mM. The relaxation times of the first intramolecular mode increase as the added salt concentration is lowered. The dependence of the translational diffusion coefficient D on the added salt concentration is not very large, although it exhibits a maximum for all three fragments. The maximum is interpreted as the consequence of two opposing effects—the stiffening of the molecule that produces an increase of the size (decrease of D) as the added salt concentration is lowered, and the coupling of the diffusion of the DNA through the electrostatic forces to the motion of the small and other polyions in the solution that results in an increase of its mobility (increase of D). The increase of the slowest intramolecular relaxation times as the salt concentration is lowered is interpreted in terms of a theory relating this time to the mean-squared radius of gyration of the molecule. © 2000 American Institute of Physics.

Notes: Accurate full-dimensional quantum mechanical calculations are reported for the CH4+H→CH3+H2 reaction employing the Jordan–Gilbert potential energy surface. Benchmark results for the thermal rate constant and the cumulative reaction probability are presented and compared to classical transition state theory as well as reduced dimensionality quantum scattering calculations. The importance of quantum effects in this system is highlighted. © 2000 American Institute of Physics.

Notes: A theory is developed for the deliquescence of small particles that incorporates the effect of surface phenomena on the deliquescence process. The theory is based on rudimentary surface thermodynamics since we are primarily interested in the orders of magnitude of the various effects. Applying the theory to a generic crystal with properties similar to NaCl, in water vapor, we find that the surface tension has a significant effect for crystals with a radius smaller than 10−5 cm. We suggest an experiment for the determination of the surface tension of soluble crystals that utilizes the deliquescence of these small particles. Serendipitously, such an experiment has just been performed by another group of authors. The qualitative analysis of this experiment should eventually be accompanied by a further development of the rigorous thermodynamic theory of solid surfaces. © 2000 American Institute of Physics.

Notes: Channel-facilitated transport of metabolites across biological membranes results in excess noise in the current carried by small ions. This noise originates from fluctuations of the number of metabolite molecules in the channel due to their diffusion. We have carried out a theoretical study of particle number fluctuations in a cylindrical pore. First, we obtain the power spectral density of these fluctuations as a function of pore length and radius, as well as the diffusion constants of the particle in the pore and in the bulk, in the absence of particle–pore interactions. We then perform three-dimensional Brownian dynamics simulations that show excellent agreement with the analytical result. Finally, we demonstrate that explicit expressions for the low-frequency limit of the spectral density can be found even when the particle interacts with the pore. © 2000 American Institute of Physics.

Notes: The kinetics of crystallization in nonadecylcyclohexane, an asymmetric alkane, was studied using x-ray scattering, differential scanning calorimetry (DSC), and optical microscopy. A transient mesophase was found with a crystallization/melting temperature 13 °C below the melting point of the stable crystal phase. In a bulk sample, nucleation from the melt proceeds though the transient mesophase, which subsequently converts to the stable phase with rather slow kinetics. When quenched to low temperatures, the mesophase does not anneal and can be described as a two-dimensional (2D) Ising-like glass with regard to the up-down orientation of the asymmetric molecules. A second metastable mesomorph was also detected at low temperatures. Using an emulsified sample, homogeneous nucleation was also shown to proceed through the transient phase. The crystal structure of the transient phase is determined. © 2000 American Institute of Physics.

Notes: We have investigated theoretically the initial reaction of nitric oxide (NO) with the Si(001)(2×1) surface, followed by N and O insertion into the silicon film during the initial growth of the oxynitride film. We use quantum chemical [ab initio and density functional theory (DFT) cluster approach] and solid state physics (DFT with periodic boundary conditions) computational methods. Our study suggests a low barrier reaction path for NO decomposition on the Si(100)(2×1) reconstructed silicon surface. © 2000 American Institute of Physics.

Notes: The adsorption mechanism of horse spleen apoferritin on smooth Si(Ti)O2 surfaces was investigated by means of optical wave guide lightmode spectroscopy (OWLS) as well as with atomic force microscopy (AFM), for which images of high resolution were obtained on muscovite mica surfaces. By the use of both experimental methods, the adsorption process could be studied from a kinetic as well as from a statistical thermodynamics point of view. This approach allowed to test the hypothesis of the occurrence of a particular type of deposition mechanism, namely the random sequential adsorption (RSA), by evaluating all the requirements that should be fulfilled in such a process. Only the requirement relative to the kinetics of the adsorption process, and subsequently, the estimation of the surface coverage at saturation is fulfilled by our experiments. From the fit of the theoretical kinetic equations corresponding to the RSA model to the experimental adsorption kinetics we find that the apoferritin molecules occupy an area of 140±30 nm2, in agreement with the values found by counting the number of particles per unit area in the AFM experiments and also with the saturation level of the adsorption isotherm. From our experiments we found that the evolution of the surface coverage close to saturation did not follow the expected power law evolution with time in the framework of the RSA model. Moreover, the dependence of the density fluctuations on the sub-surface area in the AFM image is not consistent with the expected evolution obtained by computer simulations based on the RSA model. These results emphasize the difficulty to study the adsorption mechanism of proteins at solid—liquid interfaces in the framework of any given adsorption model. © 2000 American Institute of Physics.

Notes: Atomic force microscopy is used to show that production of surface defects at the interface of rubbing solids is an important mechanism of energy dissipation in friction. Using mica and Si-tips, we demonstrate that defects produced by the rupture of Si–O bonds at the surface, which are not visible in contact mode AFM images, have a noticeable contribution to friction. When defects accumulate beyond a critical concentration, they grow to form visible wear scars ∼2 Å deep at first and deeper holes later. The contribution of defect production to friction is explained by a simple model, which is based on the stress-induced enhancement of the rate of thermal defect production. © 2000 American Institute of Physics.

Notes: A new formula, EL−E∝(L+3/4)−3, to extrapolate energies, EL (that arise when the basis set is truncated at a finite angular momentum quantum number, L) to the limit, E, is derived and applied to the computation of the pair potential of He. Large basis sets up to d-aug-cc-pV5Z and -6Z are used, and in addition, a new cc-pV7Z set is presented. The full-CI is approximated using the "multireference averaged coupled-pair functional" (MR-ACPF) with 121 references. The calculated molecular constants of He2 are in excellent agreement with those recently obtained with r12-MR-ACPF [R. J. Gdanitz, Mol. Phys. 96, 1423 (1999)], but they agree only fairly with the complete-CI estimate of van Mourik and Dunning [J. Chem. Phys. 111, 9248 (1999)]. The potential of Komasa [J. Chem. Phys. 110, 7909 (1999)] which has been calculated with the "exponentially correlated Gaussians" method does not give a bound state. The sensitivity of the molecular constants 〈R〉 and D0 to errors of the interaction potential at different distances is estimated by perturbing the potential by Gaussian functions. The region of 5(approximately-less-than)R/a0(approximately-less-than)9 is found to be most sensitive. From this analysis, doubts arise that recent calculations (including the present one) are accurate enough to allow the molecular constants to be determined to better than (approximate)10%. © 2000 American Institute of Physics.

Notes: The centroid molecular dynamics technique is applied to the case of chloride–water clusters to estimate their finite temperature quantum vibrational structure. We employ the flexible RWK2 water potential [J. R. Reimers, R. O. Watts, and M. L. Klein, Chem. Phys. 64, 95 (1982)] and the parametrization of a chloride–water interaction potential of Dorsett, Watts and Xantheas [J. Phys. Chem. A 103, 3351 (1999)]. We then investigate the temperature-dependent vibrational structure (infrared spectra). We find that the centroid molecular dynamics technique is capable of recovering a majority of the red shift associated with hydrogen bonding. © 2000 American Institute of Physics.

Notes: A new implementation of the approximate coupled cluster singles and doubles method CC2 is reported, which is suitable for large scale integral-direct calculations. It employs the resolution of the identity (RI) approximation for two-electron integrals to reduce the CPU time needed for calculation and I/O of these integrals. We use a partitioned form of the CC2 equations which eliminates the need to store double excitation cluster amplitudes. In combination with the RI approximation this formulation of the CC2 equations leads to a reduced scaling of memory and disk space requirements with the number of correlated electrons (n) and basis functions (N) to, respectively, O(N2) and O(nN2), compared to O(n2N2) in previous implementations. The reduced CPU, memory and disk space requirements make it possible to perform CC2 calculations with accurate basis sets on large molecules, which would not be accessible with conventional implementations of the CC2 method. We present an application to vertical excitation energies of alkenes C2nH2n+2, for n=1–12, and report results for the lowest lying dipole-allowed transitions for the TZVPP basis sets, which for n=12 contain 1108 basis functions. Comparison with conventional CC2 results for the smaller alkenes show that for CC2 ground state energies and for excitation energies of valence states, the error due to the RI approximation is negligible compared to the usual basis set error, if auxiliary basis sets are used, which have been optimized for MP2 energy calculations. © 2000 American Institute of Physics.

Notes: The nine elements of chemical shielding tensors contain important information about local structure, but the extraction of that information is difficult. Here we explore a semiempirical method that has the potential for providing relatively accessible structural correlations. The approach entails approximating the field-induced electron current density as entirely perpendicular to the applied field. This has two interesting consequences. (1) The resulting shielding tensor is perfectly symmetric. Thus, asymmetry in a shielding tensor is an indication of current density that is not orthogonal to the applied field. (2) The orientation dependence of the chemical shielding at a point of interest is related explicitly to the isotropic average of the chemical shielding at every point in the surrounding region. This suggests a relatively simple relationship between the orientation dependence of the chemical shielding and the molecular structure. Good correlation with experimental tensors is obtained with just one or two adjustable parameters in several series of compounds, including silicates, phosphates, hydrogen bonds, carboxyls, and amides. As expected, the results indicate that for a given center, the contribution to the shielding anisotropy that is associated with each bonded neighbor increases as the number of electrons at either the center or the neighbors increases. © 2000 American Institute of Physics.

Notes: It has been found that independent parameters in the variation of a one-electron density matrix (DM) for closed-shell systems are elements of its unitary transformed matrix and, in a special case, reduce to the rotation parameters that connect the occupied and virtual orbital spaces in the exponential transformed self-consistent field method. To obtain the unitary matrix of transformation, a simpler method of orthogonalizing the column vectors of the DM has been proposed instead of its diagonalization. An iterative method has been formulated to determine these independent parameters. Several test calculations using this method reproduced the results using the Hartree–Fock–Roothaan method. © 2000 American Institute of Physics.

Notes: A quaternion formulation is used to derive an algorithm for performing calculations on molecular clusters using the quantum diffusion Monte Carlo method. It is assumed that the monomers in the cluster rotate and translate as rigid bodies. The algorithm is tested on the water dimer and the benzene–water cluster. Comparison with dissociation energies and rotational constants obtained with other methods illustrates the accuracy of the algorithm. © 2000 American Institute of Physics.

Notes: An analytical set of field-induced coordinates (FICs) is defined. It is shown that, instead of 3N−6 normal coordinates, a relatively small number of FICs is sufficient to describe the vibrational polarizability and hyperpolarizabilities due to nuclear relaxation. The fact that the number of FICs does not depend upon the size of the molecule leads to computational advantages. A method is provided for separating anharmonic contributions from harmonic contributions as well as effective mechanical from electrical anharmonicity. Hartree–Fock calculations on a dozen representative conjugated molecules illustrate the procedures and indicate that anharmonicity can be very important. Other potential applications including the determination of zero-point vibrational averaging corrections are noted. © 2000 American Institute of Physics.

Notes: We have studied folding mechanisms of three small globular proteins: crambin, chymotrypsin inhibitor 2 (CI2), and the fyn Src Homology 3 domain (SH3) which are modeled by a Go-type Hamiltonian with the Lennard-Jones interactions. It is shown that folding is dominated by a well-defined sequencing of events as determined by establishment of particular contacts. The order of events depends primarily on the geometry of the native state. Variations in temperature, coupling strengths, and viscosity affect the sequencing scenarios to a rather small extent. The sequencing is strongly correlated with the distance of the contacting amino acids along the sequence. Thus α helices get established first. Crambin is found to behave like a single-route folder, whereas in CI2 and SH3 the folding trajectories are more diversified. The folding scenarios for CI2 and SH3 are consistent with experimental studies of their transition states. © 2000 American Institute of Physics.

Notes: The formation of clouds resulting from the homogeneous condensation of vapor phase diluted in a background or carrier gas was studied numerically. The effect of the background gas on the nucleation process in a cloud chamber heated from below is discussed. The computations were performed using 1-propanol as the condensable gas and helium, hydrogen, nitrogen, and argon, respectively, as carrier gases. Results of the simulation conducted show that large differences appear in the cloud formation when operating with hydrogen or helium, and with argon or nitrogen for which the onset of convective motions are predicted. Therefore, the isothermal patterns and streamlines are similar to those obtained in the case of Rayleigh–Bénard instabilities. The influence of the thermal Rayleigh number on the nucleation process is also considered, and it is shown that supersaturation isolines exhibit complex distortions for supercritical thermal Rayleigh numbers. As a consequence, only small zones of important nucleation rate are observed. © 2000 American Institute of Physics.

Notes: The radial profiles of the mean force and corresponding potential of mean force for the Cl−(centered ellipsis)H3O+ pair are determined by constraint molecular dynamics of an infinitely dilute near-critical aqueous solution, as described by the SPC/E water model and either the Gertner–Hynes or the Kusaka et al. hydronium model. These profiles are used to test the prediction of a continuum primitive model, and to predict the ion-pair association constant. The reliability of these intermolecular potential models is assessed by comparing the predicted association constants with those determined experimentally by conductance and solubility measurements. This comparison suggests that the most accurate experimental data available for the association constant of HCl fall between the predictions of the two models, and tends to support the superiority of the Gertner–Hynes over the Kusaka et al. hydronium model. Moreover, the simulation results allow a quick test of the reliability of the simple continuum dielectric model to represent the solvation behavior of the ion-pair in solution. © 2000 American Institute of Physics.

Notes: The mixed quantum-classical Liouville equation is derived from a semiclassical perspective starting from the full quantum Schrödinger equation. An asymptotic numerical scheme for solving the equation is discussed which relies on propagating swarms of interacting "threads" which represent the density matrix or other observable. It is demonstrated that this "multithreads" method performs extremely well on simple one-dimensional model systems designed to test nonadiabatic molecular dynamic methods, yielding essentially exact results for a variety of models. © 2000 American Institute of Physics.

Notes: The phase behavior of a system composed of spherical particles with a monomodal size distribution is investigated theoretically within the context of the van der Waals approximation for polydisperse fluids. It is shown how the binodals, spinodals, cloud-point and shadow curves as well as all the (polydispersity induced) critical points can be obtained for a variety of interaction potentials. The polydispersity induced modifications of the phase diagram (even for a polydispersity index I as small as I(approximate)1.01) should be observable in some colloidal dispersions. © 2000 American Institute of Physics.

Notes: In this paper we first develop an approximate theory for the leading order concentration dependence of the sedimentation coefficient for rodlike colloids/polymers/macromolecules. To first order in volume fraction cursive-phi of rods, the sedimentation coefficient is written as 1+αcursive-phi. For large aspect ratios L/D (L is the rod length, D its thickness) α is found to vary like ∝(L/D)2/ln(L/D). This theoretical prediction is compared to experimental results. Then, experiments on fd virus are described, both in the isotropic and nematic phase. First-order in concentration results for this very long and thin (semiflexible) rod are in agreement with the above-mentioned theoretical prediction. Sedimentation profiles for the nematic phase show two sedimentation fronts. This result indicates that the nematic phase becomes unstable with the respect to isotropic phase during sedimentation. © 2000 American Institute of Physics.

Notes: We analyze the growth of a droplet of a stable phase (a plane wave front in one-dimensional case) under strong supercooling in the framework of the Landau–Ginzburg potential for both a conserved and a nonconserved order parameter coupled to thermal diffusion. When the coupling between changes in temperature and the order parameter is strong enough, and the heat conductivity is sufficiently small, the following nontrivial phenomenon may occur: the latent heat released with the growth of a droplet is taken away very slowly, and the local overheating transforms the initially overcooled state into an overheated one. As a result, the rate of droplet growth decreases, and even may change its sign, so that droplets of radius larger than the critical one, nevertheless, may shrink. For the steady-state growth in the one-dimensional case this phenomenon was analyzed analytically, while beyond the steady regime and in the two-dimensional case, the equations were solved numerically. The presence of a thermal bath with heat loss through thermal dissipation narrows the areas of occurence of this phenomenon. © 2000 American Institute of Physics.

Notes: The morphology and the corresponding rheological properties of phase separating binary mixtures under shear flow are studied by computer simulation based on the modified time-dependent Ginzburg–Landau (TDGL) model. In order to investigate the hydrodynamic effect, model H in three dimensions has been used to simulate the phase separation of binary fluids under shear flow. For the sake of comparison, the simulation has also been performed based on simple binary solid model (model B). It is found that, for deep and critical quench, the domain grows faster and the domain anisotropy is lower in binary fluids due to the internal flow field induced by hydrodynamic interaction. For deep and off-critical quench, the internal flow field makes the elongated domain quickly relax to their original spherical shape before they are mutually contacted each other. Thus, it reduces the domain merging probability. It is also found that, for deep and critical quench, there are two peaks appeared in the shear viscosity as a function of shear strain at low shear rate, which agrees with the experimentally observations quite well. For shallow quenching, the broader interfaces suppress the internal flow caused by hydrodynamic interaction and thus the difference between binary solids and binary fluids is small. All these observed unique characters have been explained according to the hydrodynamic interaction and the relaxation rate of the deformed interface.© 2000 American Institute of Physics.