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  • Articles  (98,686)
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  • 1
    Electronic Resource
    Electronic Resource
    Semiclassical surface-hopping approximations for the calculation of solvent-induced vibrational-relaxation rate constants (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7520-7527 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Approximate schemes for the calculation of the rates of transitions between vibrational states of a molecule due to the interactions with a solvent are devised based on a rigorous, general semiclassical surface-hopping formalism developed earlier. The formal framework is based on an adiabatic separation of time scales between the fast molecular vibrations and the relatively slow bath motions. (The bath is composed of the solvent degrees of freedom plus all the molecular degrees of freedom other than vibrations.) As a result, the dynamics of the system are described in terms of bath motions occurring on adiabatic vibrational-energy surfaces, which are coupled by a nonadiabatic interaction. The time-dependent vibrational transition probability is evaluated by propagating the canonical density of the system, with the molecule in the initial adiabatic vibrational state, forward in time, and then projecting it onto the final adiabatic vibrational state of interest. The temporal evolution of the density is carried out with a semiclassical surface-hopping propagator, in which the motion of the bath on an adiabatic vibrational surface is described in terms of the familiar (adiabatic) semiclassical propagator, while transitions are accounted for in terms of instantaneous hops of the bath paths between the adiabatic vibrational surfaces involved, with an integration over all possible hopping points.Energy is conserved in the hops, and the only component of momentum that changes is the one along the nonadiabatic coupling vector. When the nonadiabatic interaction is taken into account to first order, the transition probability is predicted to become linear in the long-time limit. Various methods for extracting the relaxation rate constant in this limit are presented, and a simple model system with a one-dimensional bath is employed to compare their practical efficiency for finite time. In addition, this system is used to numerically demonstrate that local approximations for the adiabatic vibrational surfaces and the nonadiabatic coupling yield accurate results, with great reduction of the amount of computation time. Since a local approximation for the vibrational surfaces makes an N-dimensional problem separable into N effectively one-dimensional ones, this treatment is seen to be more generally applicable to realistic systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468245
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  • 2
    Electronic Resource
    Electronic Resource
    Theoretical study of Ca(4s5p 1P)→Ca(4s5p 3P) energy transfer in collisions with He. Initial and final state alignment (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7554-7565 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we present the results of close-coupling calculations of the cross sections for Ca(4s5p 1P)→Ca(4s5p 3P) energy transfer in collisions with He, based on new potential energy curves. Particular attention is devoted to the simulation of the recent experiment of Smith and co-workers [J. Chem. Phys. 96, 8212 (1992)], in which, for the 1P→3P2 transfer both initial and final alignment are controlled with respect to the initial relative velocity vector of the two partners Vrel. The calculated polarization ratios (σ⊥/σ(parallel)), defined as the ratio of the cross sections for the 1P→3P transfer summed over final levels for initial alignment of the 5p orbital perpendicular and parallel to Vrel are in good agreement with the experimental results. The theoretical cross sections for the 1P→3P2 transfer determined in the so-called collision frame (σj1m1m1→j2m2m2), where the axis of quantization is taken along Vrel are in good agreement with the experiment in the case of initial perpendicular excitation, but show quantitative and qualitative disagreement in the case of initial parallel excitation. We find that relative populations into the final m2 levels depend strongly on the initial orientation of the 5p orbital with respect to the initial collision plane. However, we show, that in the case where the 5p orbital lies in the initial collision plane, the relative populations in the final m2 components of the 3P2 state depend exclusively on interactions among the exit channels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468250
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  • 3
    Electronic Resource
    Electronic Resource
    On the energy dependence of the steric effect in atom–molecule reactive scattering. I. A quasiclassical approach (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7592-7602 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Experimental studies have shown that the steric effect in chemical reactions can decrease (e.g., for Ba+N2O→BaO*+N2) or increase [e.g., for Ca(1D2)+CH3F→CaF*+CH3] with increasing translational energy. Decreasing (negative) energy dependences have successfully been modeled with the angle dependent line of centers model. We present a classical model in which a positive energy dependence of the steric effect is explained by an isotropic, attractive long range potential. In this "trapping'' model we assume the reaction—apart from a cone of nonreaction at one side of the molecule—to be barrierless. This model shows that a positive energy dependence of the steric effect is not indicative of reorientation of the molecule, as has been suggested in the literature. Rather, the positive or negative energy dependence of the steric effect is shown to correlate with the absence or presence of a barrier to reaction and an attractive or repulsive long range potential. For the reorientation effects which occur in the case of anisotropic potentials, we consider the application of the standard quasiclassical trajectory (QCT) method and we introduce a modified QCT method. We argue that the latter is more suitable for the computation of the orientation dependent reactive cross section.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468253
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  • 4
    Electronic Resource
    Electronic Resource
    Hydrated proton clusters and solvent effects on the proton transfer barrier: A density functional study (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7633-7642 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The density functional calculations using the Perdew nonlocal corrections to exchange and correlation have been carried out for a sequence of hydrated proton clusters. The optimized structures were obtained up to H13O+6. It is found that H3O+ is indeed the central unit in all the lowest energy structures we found. Our results support the argument that the structure with a four-coordinate first solvation shell is very unlikely in small hydrated proton clusters. The density functional calculations with the Perdew nonlocal corrections to exchange and correlation give somewhat shorter hydrogen bond lengths, but slightly longer chemical bond lengths as compared with the post-Hartree–Fock calculations. The harmonic vibrational frequencies and IR intensities of various vibrational modes have been generated for all the structures optimized. Results for small clusters are compared with the high resolution experimental spectroscopy studies of Yeh et al. and Begemann et al. Results for larger clusters are used to interpret the low resolution spectra of Schwartz. Very good accord with experimental results is obtained. The solvent effects on proton transfer energy barriers in clusters have been studied by designing a few model systems. The barrier is found to be very sensitive to the solvent configurations. When the solvent water is replaced by the classical partial charge model, a significant change of the barrier is observed, indicating that a quantitative treatment will ultimately require a good pseudopotential to properly account for the quantum nature of the solvent. A combined density functional and molecular dynamics simulation was used to calculate the proton transfer energy and free energy barrier in aqueous solution. The barrier is found to be 3 kcal/mol higher than in gas phase. Very large solvent fluctuation is observed which may have a significant influence on the reaction rate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468256
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  • 5
    Electronic Resource
    Electronic Resource
    Low-temperature proton transport in clathrates (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7672-7682 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The impedance spectra are measured for protonated and deuterated clathrates, HClO4⋅5.5H2O and DClO4⋅5.5D2O, between 10 and 300 K. The conductance is investigated between 80 K and room temperature and the dielectric constant between 10 and 120 K. The data show deviation from the Arrhenius behavior of conductance in the low-temperature regime. A description of proton conductivity is developed on the basis of quantum theory of an elementary act of proton tunneling between donor–acceptor sites interacting with environmental fluctuations. Several models of the elementary act are considered. The mechanism, most consistent with the obtained data, incorporates—strong coupling of the proton with local vibrational modes of the closest environment and system diabatic transitions along these vibrational "coordinates''—fluctuations of the tunneling barrier for the proton. At low temperatures the motion along the vibrational coordinates is no longer purely classical and the slow mode tunneling takes place. The latter gives rise to a curvature in the Arrhenius plots apprehended as a decrease of the apparent activation energy at lower temperatures. The observed isotope effect is in line with the lower deuteron tunneling probability due to doubling the mass, though other parameters may also be affected by deuteration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468260
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  • 6
    Electronic Resource
    Electronic Resource
    Optical potential coupled to discrete variable representation for calculations of quasibound states: Application to the CO(B 1Σ+–D'1 Σ+) predissociating interaction (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7580-7591 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The optical potential method initially proposed by Jolicard and Austin in the context of the stabilization method is reviewed here and used with the accurate and the efficient discrete variable representation method to obtain the energies and the widths (respectively, the real and the imaginary part of the resonance energies) of rovibrational predissociated states of diatomic molecules. In this method the resonances for an n coupled states problem are obtained by a direct diagonalization of the Hamiltonian matrix in the diabatic representation. This Hamiltonian matrix is directly evaluated in the discrete variable representation using the Fourier grid Hamiltonian method proposed by Marston and Balint-Kurti. In this approach, two optical potentials are tested and used here to impose the asymptotic behaviors of the boundary conditions which are compatible with the resonance states. The method is exemplified for the B 1Σ+–D'1 Σ+ Rydberg–valence predissociating interaction in the CO molecule.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468252
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  • 7
    Electronic Resource
    Electronic Resource
    A self-adaptive multilevel finite element method for the stationary Schrödinger equation in three space dimensions (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7643-7650 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An error controlled finite element method (FEM) for solving stationary Schrödinger equations in three space dimensions is proposed. The method is based on an adaptive space discretization into tetrahedra and local polynomial basis functions of order p=1–5 defined on these tetrahedra. According to a local error estimator, the triangulation is automatically adapted to the solution. Numerical results for standard problems appearing in vibrational motion and molecular structure calculations are presented and discussed. Relative precisions better than 1e-8 are obtained. For equilateral H++3, the adaptive FEM turns out to be superior to global basis set expansions in the literature. Our precise FEM results exclude in a definite manner the stability or metastability of equilateral H++3 in its ground state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468257
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  • 8
    Electronic Resource
    Electronic Resource
    Pressure dependence of magnetic coupling in ionic solids from ab initio cluster model calculations (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7683-7685 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dependence of the magnetic coupling constant, J, with the pressure has been studied by an ab initio cluster model approach in a typical ionic solid such as KNiF3. By computing J at different values of the lattice parameter R, we predict a power law of the form ||J||≈R−n with 10.5〈n〈13.3 in good agreement with the suggested experimental value. This behavior is found even for the simplest Ni2F cluster model described by a complete active space configuration interaction wave function in which the active orbitals correspond to the open shell of each Ni2+ cation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468261
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  • 9
    Electronic Resource
    Electronic Resource
    Calculation of rovibronic structures in the lowest nine excited 1Σ+g+1Πg+1Δg states of H2, D2, and T2 (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7692-7706 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have calculated the rovibronic structures which arise from the simultaneous radial and angular couplings among the lowest nine excited 1Σ+g, 1Πg, and 1Δg states in H2, D2, and T2. Using ab initio potential curves, adiabatic corrections, and nonadiabatic coupling functions wherever possible, we have solved the coupled vibronic equations for all bound states below the 1s+2l dissociation limit up to J=5 in H2 and D2, and up to J=7 in T2. The good agreement of the bulk of the data with spectroscopic results enables us to eliminate spurious assignments in old spectroscopic work (e.g., all old assignments involving the 4s O 1Σ+g state are spurious), and to make numerous new assignments. We present updated tables of spectroscopic term values for H2 and D2. The absolute energy errors of the ab initio Born–Oppenheimer energies around the minima of the potential curves appear to be at most of the order of the radiative energy corrections ((approximately-equal-to)0.1 cm−1) in the states 1σg2sσg E and (1σu)2 F, less than 1 cm−1 in the states 3dσ G, 3s H, and 3dπ I, less than 2 cm−1 in the 4s O state, and approximately 4 cm−1 in the 4dσ P state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468263
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  • 10
    Electronic Resource
    Electronic Resource
    Computer simulations of cesium–water clusters: Do ion–water clusters form gas-phase clathrates? (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 7873-7881 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure and energetics of cesium ion–water clusters have been investigated using classical molecular dynamics computer simulations and a polarizable interaction model. Recent experiments by Selinger and Castleman [J. Phys. Chem. 95, 8442 (1991)] indicate that the mass-spectral distributions for these clusters exhibit "magic number'' oscillations at temperatures below approximately 160 K. The observed behavior of this and related charged clusters is commonly attributed to the formation of clathratelike cage structures around a central ionic species. The relationship between the structural and energetic properties of cesium ion–water clusters is reported here as a function of temperature for clusters with between 18 and 22 water molecules. The clusters exhibit solidlike dynamical behavior at kinetic temperatures below about 170 K, and liquidlike behavior at higher temperatures. A thorough analysis of energy minimized (0 K) structures indicates that the most stable clusters consist of water cages surrounding the cesium ion. These cages are related to the proposed clathratelike structures but contain additional 4- and 6-membered water rings and fewer 5-membered rings. The calculated global energy minima exhibit an energetic alternation with cluster size that is consistent with the experimentally observed mass-spectral distributions. In contrast, in the liquidlike regime there are only minor variations in calculated structural and dynamical properties as a function of cluster size. In addition, there is no statistically significant size dependence for the cluster binding energies in the liquidlike regime that might be correlated with experimental data. These results suggest that magic number stability in ion–water clusters may occur only at "low'' energies in the solidlike cluster regime.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468213
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