Molecular Dynamics Simulations Of Glasses Research Articles

Selecting reliable interatomic potentials for classical molecular dynamics simulations of glasses: The case of amorphous SiO2

This paper presents an approach to judge the quality of classical interatomic potentials used in molecular dynamics simulations of glasses. The static structure and dynamical properties of amorphous SiO2 were simulated by classical molecular dynamics using a series of well known interatomic potentials. Theoretical X-ray and neutron structure factors and effective neutron-weighted vibrational density of states of amorphous SiO2 were computed from the obtained atomistic configurations and quantitatively compared to experimental results. The interatomic potential which best reproduced the experimental X-ray and neutron scattering data severely failed to reproduce the experimental vibrational density of states of amorphous SiO2. It is found that only the potential developed by van Beest, Kramer, and van Santen (BKS) was able to adequately reproduce both static structure and dynamical properties of amorphous SiO2. Thus, the fact that an interatomic potential is able to properly reproduce static structures of amorphous systems should not be considered as a basis to use this potential to simulate other properties of these systems.

Cooling rate and stress relaxation in silica melts and glasses via microsecond molecular dynamics.

We have conducted extremely long molecular dynamics simulations of glasses to microsecond times, which close the gap between experimental and atomistic simulation time scales by two to three orders of magnitude. Static, thermal, and structural properties of silica glass are reported for glass cooling rates down to 5×10(9) K/s and viscoelastic response in silica melts and glasses are studied over nine decades of time. We present results from relaxation of hydrostatic compressive stress in silica and show that time-temperature superposition holds in these systems for temperatures from 3500 to 1000 K.

A Computational Tool for the Prediction of Crystalline Phases Obtained from Controlled Crystallization of Glasses

An automatic tool (named CLUSTER) for the prediction of the most probable crystal phases that can separate from glasses has been developed. The program analyzes the output of molecular dynamics simulations of glasses or glass ceramics, systematically sampling the ratios of the ions in different portions of the simulation box and comparing them to the stoichiometric ratio of compositionally equivalent crystalline phases retrieved from a crystal structure database. The efficacy of the similarity index elaborated has been judged by comparing the results obtained with the crystal phases identified by XRD analysis after thermal treatment in a series of multicomponent potential bioactive glasses and glass ceramics for which the advantages of rational-designed erosion-controlled release is straightforward.

Collectivity of motion in undercooled liquids and amorphous solids

The motion of the structure determining components is highly collective, both in amorphous solids and in undercooled liquids. This has been deduced from experimental low temperature data in the tunneling regime as well as from the vanishing isotope effect in diffusion in glasses and undercooled liquids. In molecular dynamics simulations of glasses one observes that both low frequency resonant vibrations and atomic jumps are centered on more than 10 atoms which, in densely packed materials, form chain-like structures. With increasing temperature the number of atoms jumping collectively increases. These chains of collectively jumping atoms are also seen in undercooled liquids. Collectivity only vanishes at higher temperatures. This collectivity is intimately related to the dynamic heterogeneity which causes a non-Gaussianity of the atomic displacements.

Molecular dynamics simulations of glasses

We present MD simulations of two glass-forming structures (Se and SiO2) at temperatures above and below the glass transition. In both structures we find the relaxations to be localized on a small number of atoms. However, both the number of atoms involved in jumps and the jump-length grow with temperature. In Se the relaxing units form typically chain-like structures. In the simulations of SiO2-glasses we observe that the displacement of the oxygen is larger than the corresponding Si contribution. In these cases the relaxations occur predominantly at atoms which are topological defects (under- or rarely over-coordinated).