Simona Ispas

New fitting scheme to obtain effective potential from Car-Parrinello molecular-dynamics simulations: Application to silica

A fitting scheme is proposed to obtain effective potentials from Car-Parrinello molecular-dynamics (CPMD) simulations. It is used to parameterize a new pair potential for silica. MD simulations with this new potential are done to determine structural and dynamic properties and to compare these properties to those obtained from CPMD and a MD simulation using the so-called BKS potential. The new potential reproduces accurately the liquid structure generated by the CPMD trajectories, the experimental activation energies for the self-diffusion constants and the experimental density of amorphous silica. Also lattice parameters and elastic constants of α-quartz are well reproduced, showing the transferability of the new potential.

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

Amorphous silica modeled with truncated and screened Coulomb interactions: A molecular dynamics simulation study

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

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

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.

First-principles study of a sodium borosilicate glass-former. I. The liquid state

We show that finite-range alternatives to the standard long-range pair potential for silica by van Beest et al. [Phys. Rev. Lett. 64, 1955 (1990)] might be used in molecular dynamics simulations. We study two such models that can be efficiently simulated since no Ewald summation is required. We first consider the Wolf method, where the Coulomb interactions are truncated at a cutoff distance rc such that the requirement of charge neutrality holds. Various static and dynamic quantities are computed and compared to results from simulations using Ewald summations. We find very good agreement for rc approximately 10 A. For lower values of rc, the long-range structure is affected which is accompanied by a slight acceleration of dynamic properties. In a second approach, the Coulomb interaction is replaced by an effective Yukawa interaction with two new parameters determined by a force fitting procedure. The same trend as for the Wolf method is seen. However, slightly larger cutoffs have to be used in order to obtain the same accuracy with respect to static and dynamic quantities as for the Wolf method.

Nanoindentation of the pristine and irradiated forms of a sodium borosilicate glass: Insights from molecular dynamics simulations

The structural properties of glassy and liquid sodium tetrasilicate Na2Si4O9 are studied by Car–Parrinello molecular dynamics simulations. The results are compared to those obtained for the same samples by classical molecular dynamics simulations using a Kramer, van Beest and van Santen-type potential. We note that, in the Na2Si4O9 liquid, the ab initio description gives a higher number of defects than the classical one. In particular, comparison between the coordinations, the pair correlation functions and the angle distributions given by the two descriptions are presented. In the Na2Si4O9 glass, the details of the local structure are found to be different in the classical and ab initio descriptions. We show that these differences are related to the modifications of the atomic charges due to the introduction of Na atoms, which are not taken into account in the classical simulations.

Behavior of sodium borosilicate glasses under compression using molecular dynamics

We use ab initio simulations to study the static and dynamic properties of a sodium borosilicate liquid with composition

On a mixed Neumann-Robin boundary value problem in electrical impedance tomography

3{\mathrm{Na}}_{2}\mathrm{O}--{\mathrm{B}}_{2}{\mathrm{O}}_{3}--6{\mathrm{SiO}}_{2}

New optimization scheme to obtain interaction potentials for oxide glasses

, i.e., a system that is the basis of many glass-forming materials. In particular, we focus on the question how boron is embedded into the local structure of the silicate network liquid. From the partial structure factors we conclude that there is a weak nanoscale phase separation between silicon and boron and that the sodium atoms form channel-like structures as they have been found in previous studies of sodosilicate glass-formers. Our results for the x-ray and neutron structure factor show that this feature is basically not detectable in the former but should be visible in the latter as a small peak at small wave vectors. At high temperatures we find a high concentration of threefold coordinated boron atoms which decreases rapidly with decreasing

Some boundary problems in electrical impedance tomography

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