Ab Initio Molecular Dynamics Simulation of Liquid Ga_xAs_{1-x} Alloys

We report the results of ab initio molecular dynamics simulations of liquid Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600 K, just above the melting point of GaAs. The liquid is predicted to be metallic at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity maximum near x = 0.5, consistent with the Faber-Ziman expression. The electronic density of states is finite at the Fermi energy for all concentrations; there is, however, a significant pseudogap especially in the As-rich samples. The Ga-rich density of states more closely resembles that of a free-electron metal. The partial structure factors show only a weak indication of chemical short-range order. There is also some residue of the covalent bonding found in the solid, which shows up in the bond-angle distribution functions of the liquid state. Finally, the atomic diffusion coefficients at 1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in Ga_{0.2}As_{0.8}.Comment: 29 pages, 10 eps figures, accepted for publication in Phys. Rev.

RESISTIVITY OF LIQUID Ga BETWEEN 303 K AND 573 K FROM STRUCTURE FACTOR MEASURENENTS

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Temperature dependence on structure and dynamics of Bovine Pancreatic Trypsin Inhibitor (BPTI) A neutron scattering study

We have studied the influence of temperature on the structure of BPTI in solution by small angle neutron scattering. We have investigated the variation of the radius of gyration and the modification of the shape of BPTI between ambient temperature and 368 K. Results have shown an increase of the radius of gyration from 10.9 A at ambient temperature up to 13.3 A at 368 K. Global and internal dynamics of BPTI in solution were studied by quasielastic neutron scattering. The analysis of neutron data in terms of intermediate scattering function reveals two relaxation times tau(1) and tau(2) related respectively to global translational diffusive motions and to internal motions of protein. Motions of protons belonging to lateral chains of residues located at the surface of the protein have been detected. The results are compared to the recently published results concerning the influence of pressure on structure and dynamics of BPTI in solution [Appavou MS et al. Biochimica et Biophysica Acta, 1764, 2006, pp 414-423]

Quantum effects in the structural properties of supercritical He4

Neutron-diffraction measurements have been performed on supercritical fluid He4 in the 1485 K temperature range along an isochore passing close to the lower point. The quantum nature of the particle pair distribution functions is analyzed by comparing our results with classical molecular-dynamics simulations. Deviations from classical behavior are seen to persist up to T=85 K although they are much larger at lower temperatures. A measure of these deviations is obtained through an empirical parameter describing quantum delocalization around the classical positions. Limiting low-temperature behavior of this parameter is discussed and compared with direct experimental evidence of low-temperature quantum delocalization obtained from the particle momentum distribution. © 1994 The American Physical Society

Liquid-like and solid-like motions in proteins

Recent analyses of molecular dynamics simulations of hydrated C-phycocyanin suggest that the internal single-particle dynamics of this protein can be decomposed into four almost decoupled motion types: (1) diffusion of residues ("beads") in an effective harmonic potential, (2) corresponding vibrations in a local potential well, (3) purely rotational rigid side-chain diffusion, and (4) residue deformations. Each residue bead is represented by the corresponding C[α] carbon atom on the main chain. The effective harmonic residue potential can be imagined as the envelope of many local wells which are separated by small energy barriers. The residue friction matrix is assumed diagonal and the individual friction constants can be related to the density of the surrounding atoms. In this article we show that our model can be applied to lysozyme in solution as well, the only difference being that the side-chain deformations are more important and seem to be strongly correlated with the side-chain rotations. Comparing the simulated coherent scattering function of C-phycocyanin to a neutron spin-echo spectrum we show that our model can also describe collective motions in proteins at the residue level

Diffusional and vibrational properties of water confined in very thin nanoporous glasses probed by light and neutron scattering

Rayleigh-wing, Raman and quasi-elastic neutron scattering data on water confined into nanoporous glasses (silica 26 Å pores), are presented and discussed. The H-bond among water molecules and Si-OH surface groups originates strong hydration effects, that induce more stable structural environments in which the memory of collective modes (the [Math]60 cm-1 and [Math]190 cm-1 shoulders due to the HB bending and HB stretching modes of water) is lost and the O-H stretching band connected to low-density water tends to disappear. The quasi-elastic neutron response is explained in terms of two available current models : the Dianoux-Volino model and the MCT model