Magali Benoit

Tunnelling and zero-point motion in high-pressure ice

The microscopic structure of ice poses a long-standing challenge to theory. Because of their low mass, the protons in the hydrogen bonds that define the structures of crystalline ice are susceptible to quantum-mechanical effects such as tunnelling,. High pressure provides a means of controlling the length of the hydrogen bonds in order to investigate such effects. In particular, Holzapfel predicted 26 years ago that, under pressure, hydrogen bonds might be transformed from the highly asymmetric O–H···O configuration to a symmetric state in which the proton lies midway between the two oxygens, leading to a non-molecular symmetric phase of ice, now denoted as ice ‘X’. The existence of this phase has been inferred from spectroscopy, but has still not been observed directly. Here we investigate the role of quantum effects in proton ordering and hydrogen-bond symmetrization within ice at high pressure by using a simulation technique that treats both electrons and nuclei quantum-mechanically. We find that the proton-ordered structure at low pressure, with asymmetric hydrogen bonds (ice VIII), transforms on increasing pressure to a proton-disordered asymmetric phase (ice VII) owing to translational proton tunnelling. On further compression, the zero-point fluctuations lead to strongly delocalized protons and hydrogen-bond symmetrization, even though the underlying character of the proton-transfer potential remains a double well. Only at still higher pressures does the double-well potential become transformed into a single well, whereupon the protons again become increasingly localized.

Structural and electronic properties of the sodium tetrasilicate glass Na 2 Si 4 O 9 from classical and ab initio molecular dynamics simulations

The structure and the electronic properties of a sodium tetrasilicate

Signature of small rings in the Raman spectra of normal and compressed amorphous silica: A combined classical and ab initio study

({\mathrm{Na}}_{2}{\mathrm{Si}}_{4}{\mathrm{O}}_{9})

Efficient Calculation of Free Energy Differences Associated with Isotopic Substitution Using Path-Integral Molecular Dynamics.

glass were studied by combined Car-Parrinello and classical molecular dynamics simulations. The glass sample was prepared using a method recently employed in a study of a silica glass [M. Benoit et al., Euro. Phys. J. B 13, 631 (2000)]. First we generated a NS4 glass by classical molecular dynamics and then we took it as the initial configuration of a first-principles molecular dynamics simulation. In the ab initio molecular dynamics simulation, the electronic structure was computed in the framework of the Kohn\char21{}Sham density functional theory within the generalized gradient approximation using a B-LYP functional. The Car-Parrinello dynamics is remarkably stable during the considered trajectory and, as soon as it is switched on, some significant structural changes occur. The ab initio description improves the comparison of the structural characteristics with experimental data, in particular concerning the Si\char21{}O and Na\char21{}O bond lengths. From an electronic point of view, we find that the introduction of the sodium oxide in the silica network lowers the band gap and leads to a highly nonlocalized effect on the charges of the network atoms.

Structural properties of glassy and liquid sodium tetrasilicate: comparison between ab initio and classical molecular dynamics simulations

We calculate the parallel (VV) and (VH) perpendicular polarized Raman spectra of amorphous silica. Model

The vibrational dynamics of vitreous silica: Classical force fields vs. first principles

{\mathrm{SiO}}_{2}

Multiple scattering calculations of the XANES Si K-edge in amorphous silica

glasses, uncompressed and compressed, were generated by a combination of classical and ab initio molecular-dynamics simulations and their dynamical matrices were computed within the framework of the density-functional theory. The Raman-scattering intensities were determined using the bond-polarizability model and a good agreement with experimental spectra was found. We confirm that the modes associated to the fourfold and threefold rings produce most of the Raman intensity of the

The role of quantum effects and ionic defects in high-density ice

{D}_{1}

Structural and vibrational properties of a calcium aluminosilicate glass: classical force-fields vs. first-principles

and

Quantum effects on phase transitions in high-pressure ice

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