ALBERT

Notes: Combined neutron and x-ray reflectometry was used to determine mass density and hydrogen content for thin (less than 3000 A(ring)) diamondlike hydrocarbon films on polished Si (100) substrates. The introduction of "twin'' neutron reflectometry improved the accuracy of the measurements. Mass densities in the range of 1.60–1.84 g/cm3 and hydrogen contents of 31–36 at. % were determined together with the nucleus densities of hydrogen and carbon. A series of diamondlike carbon films prepared by plasma-enhanced chemical vapor deposition was investigated to analyze the effect of depositing films at different flow rates of methane precursor gas. The mass density was found to increase with flow in the range of 1–8 standard cm3/min (sccm). For higher flows the mass density was constant. The hydrogen content did not depend on the rate of flow in the interval of 2–30 sccm. Finally, results of the mass density measured in a simpler way, using a scanning electron microscope and a profilometer, were compared to the results obtained by neutron and x-ray reflectometry. © 1996 American Institute of Physics.

Notes: Aggregation of casein micelles after addition of the proteolytic enzyme chymosin has been studied by static and dynamic light scattering at three different concentrations of casein corresponding to dilutions 1:100, 1:500, and 1:1000 of native milk. The static light scattering data have been analyzed by an indirect Fourier transformation method which gives the distance distributions as a function of time. From these curves radius of gyration and an average number of casein micelles in the aggregates have been derived as a function of time. The dynamic light scattering experiments give the hydrodynamic radius as a function of time after the addition of rennet. The initial radius of gyration for the intact casein micelles is 140 nm. The corresponding hydrodynamic radius is also 140 nm. This shows that the casein micelles are not solid spheres. Inspection of a plot of relative mass versus radius of gyration for the aggregates appearing after the addition of chymosin shows that two processes take place. First extended linear aggregates are built up to a relative mass of the aggregates of about 10 and then restructuring of aggregates occurs such that increasingly compact objects are formed. Whereas the first process exhibits a relatively fast growth in size, the aggregates grow slowly in size during the second process. Further evidence of the formation of linear aggregates followed by more dense aggregates was obtained by forming the ratio between the radius of gyration and the hydrodynamic radius.This ratio increases to values of about 2.5 (indicating that linearly extended molecules are present followed) by a decrease to about 1. The log–log plot of mass versus radius of gyration is linear up to relative masses of about 10 with a slope of about 2. This extends up to sizes of 1 μm in diameter. The slope then increases to values indicating branching and thereby the formation of more compact aggregates. For relative masses below 10 and sizes below 1 μm sedimentation is unlikely to occur and information about the mechanism of aggregation can be obtained. The aggregation number as a function of time has been analyzed in terms of Smoluchowski's equations with a rate constant including both functionality and a changing barrier height as a function of the extent of proteolysis. The functionality obtained from Smoluchowski's equations is about 2.1. © 1995 American Institute of Physics.

Notes: The components of the full nuclear shielding tensor are analyzed with particular regard to the origins of the antisymmetric component of the shielding of nuclei at low-symmetry molecular sites. As an aid we propose the study and display of the shielding response vector, i.e., the nuclear shielding field per unit applied magnetic field. The analysis is based on ab initio calculations in the localized orbital-local origin method for cyclopropane, cyclopropene, ethylene oxide, ethylene imine, and diazirine, and also includes a discussion of the mechanism for the large antisymmetric component predicted for the unsaturated nuclei in cyclopropene and diazirine, both in terms of bond and lone pair contributions and in terms of a decomposition of the dominant paramagnetic contributions into molecular point group species. Display of the component of the shielding vector parallel to the applied field is shown to be a valuable alternative to the common ellipsoid representations. Display of the components perpendicular to the applied field provides a picture of the relation of the antisymmetry to molecular structure and suggests its direct observation through monitoring the perpendicular response. Finally, we show that, contrary to a recent claim, the eigensolutions of the full tensor in the presence of antisymmetry play no role in the observed anisotropies.

Notes: Absolute Raman trace scattering cross sections have been measured for gaseous propane-h8, propane-d8, 1,1,1,2,3,3,3-propane-d7, and the two rotational isomers of 1,1,2,2,3,3,3-propane-d7. A set of ∂α¯/∂Sj intensity parameters were determined from the experimental cross sections and compared with the ones obtained from quantum chemistry calculations. The anomalous intensity ratio observed for the CH stretching bands of the two conformers of CHD2CD2CD3 is explained by an unanticipated difference in the ∂α¯/∂rCH value for the two types of methyl CH bonds.

Notes: Time-dependent Hartree–Fock theory leads to two apparently quite different expressions relating matrix elements of the electronic position and momentum operators, depending upon whether a single-particle or a many-particle matrix element is used as the starting point. The two expressions are shown to be equivalent, in a complete Hartree–Fock basis set, by means of explicit use of the commutation properties of r with V and J on the one hand, and self-consistency relations and Brillouin's theorem on the other.

Notes: Density functional theory (DFT) has been applied to the study of the fluid–solid transition in systems with realistic potentials (soft cores and attractive forces): the purely repulsive WCA Lennard-Jones reference potential (LJT), the full Lennard-Jones potential (LJ) and the exponential-6 potential appropriate for helium and hydrogen. Three different DFT formalisms were used: the formulation of Haymet and Oxtoby (HO) and the new theories of Denton and Ashcroft (MWDA) and of Baus (MELA). The results for the melting pressure are compared with recent simulation and experimental data. The results of the HO version are always too high, the deviation increasing when going from the repulsive Lennard-Jones to the exponential-6 potential of H2. The MWDA gives too low results for the repulsive Lennard-Jones potential. At low temperatures, it fails for the full LJ potential while at high temperatures it is in good agreement. Including the attraction as a mean-field correction gives good results also for low temperatures. The MWDA results are too high for the exponential-6 potentials. The MELA fails completely for the LJT potential and the hydrogen exponential-6 potential, since it does not give a stable solid phase.

Notes: The Gaussian wave packet method has been developed for the simulation of processes like molecular collisions, photodissociation of molecules, and laser excitations of molecules. So far a necessary condition for an accurate result is that the fragment states are propagated accurately. We have considered one-dimensional bound states described by a Morse potential, and carried out a systematic study of the ability of the Gaussian wave packet method to propagate the stationary states. It is found that the complete set of equations of motion as derived by the minimum error method (MEM) cannot be used in practical calculations because of numerical problems. These are eliminated by the introduction of simplifications such as the independent Gaussian approximation (IGA), where each wave packet is propagated independently. The conditions for an accurate propagation within that assumption are developed, and a simple method is devised to identify those states, which are propagated accurately. This procedure may be used to investigate when the Gaussian wave packet method is appropriate for the simulation of a given process.

Notes: Three very long runs of constant temperature molecular dynamics (MD) simulations, extending over typically 2×105 time steps, have been carried out for a state just above the glass transition of equimolar soft-sphere mixtures, with 500 atoms interacting through vαβ(r) =ε(σαβ /r)12 , where σαβ =(σα +σβ )/2 , and α,β (species indices)=1, 2. The ratio of diameters σ2/σ1 was chosen to be 1.2. Jump-type atomic motions are clearly found to occur in the glassy states. A coordinated (strongly correlated) jump motion was observed, where a cluster of several atoms dynamically linked at nearest-neighbor distances jump at successive close times. These simulations demonstrate that the characteristic time scale of structural relaxation becomes very long near the glass transition, and calculated mean square displacements exhibit complicated behaviors. Thermodynamic quantities, however, such as the mean pressure, energy, and pair distribution functions, appear to behave normaly, with no significant observable relaxations for increasing time, so that their averages are well defined.

Notes: The freezing transition of "soft spheres,'' which interact via inverse-power potentials v(r)∼r−n, is systematically investigated for various values of the steepness n, using density functional theory. Two "second-order'' theories are considered, which require as only input the static pair structure of the homogeneous fluid. Both theories overestimate the densities of the coexisting fluid and solid phases, and the disagreement with computer simulation data worsens as the steepness n decreases. The two theories also predict that the fcc phase is more stable than the bcc phase for all n, which contradicts the behavior expected for soft repulsions.