Modeling Oxides, Ceramics, Zeolites and Glasses
Silica Force Fields
We have developed new interatomic potentials for the descriptions of the
various forms of silica.1
We used Morse potentials for the short range
interactions and the electrostatic interactions are taken as pure
coulombic, i.e. long ranged. The charges of the ions are taken to be
dependent. They are determined by using the Charge Equilibration method
of Rappe and Goddard. Using these new interatomic potentials we
performed structure optimizations and molecular dynamics simulations of
all silica polymorphs. We compare and contrast our results with
the potentials developed earlier by other researchers. To test the
transferability of this interaction potentials we have studied pressure
induced phase transformations such as quartz to stishovite and coesite
to stishovite under various loading rates and temperatures.
Simulation of silica polymorphs and glasses
In the past few years, considerable effort has gone into developing
force fields to use in molecular dynamics (MD) simulations of inorganic
glasses. These simulations are used to obtain molecular level insights
into the structure of glasses. However, there has been no published
systematic study detailing the impact of various MD approaches used to
generate the glass structure upon the calculated structure parameters.
In this paper, we investigate the effect of the initial structure
(randomly generated or melt obtained from crystal), annealing cycle
(initial soak temperature and time as well as cooling rate), system size
(ranging from 600 to 3240 atoms), type of dynamics (NPT vs. NVT) and
force field upon a variety calculated structural parameters of vitreous
silica. These parameters include density, radial distribution function,
bond angle distribution, and ring size distribution. The
reproducibility of these calculated parameters is also studied.
Simulation Phase transitions in Silica
Silica, SiO2, is one of the most widely studied substance, and it has some
complex and unusual properties. We
have used recently developed 2-body interaction force field
to study the structural phase transformations in
silica under various pressure loading conditions.
The specific transformations we studied are alpha-quartz to
stishovite, coesite to stishovite and fused glass to stishovite-like dense,
a dominantly six-coordinated glassy
phase. Molecular dynamics simulations are performed under constant loading
rates ranging from 0.1 GPa/ps to
1.0 GPa/ps, pressures upto 100 GPa and at temperatures 300, 500, and 700 K.
We observe the crystal to crystal
transformations to occur reconstructively, whereas it occurs in a smooth
and displacive manner from glass to a
stishovite-like phase confirming earlier conjectures.
To elucidate the shock loading experiments, we studied
the dependence of transition pressure on the loading rate and the temperature.
To assess the hysterisis effect we
also studied the unloading behavior of each transformation.
Molecular structures from alpha quartz simulation:
before phase transformation and
after the phase transformation.
Force Fields and Simulation of Aluminophosphates
Aluminophosphate zeolite is an artificial material which shows strange
hydrophilicity. We have been
investigating the reason by quantum mechanics, and found that the
hydrophilicity of this zeolite depended on the
local geometric deformation and, speculated that the site-specific
hydrophilicity might be explained by
differences in the stiffness of the local deformation determined by
the location with the pore structure. In order to
test these ideas, we construct a reliable force field based on
the new MS-Q one developed by Demiralp, Cagin,
Our force field well reproduces the experimental structure of VPI-5.
Extensions and Current Work
Recent efforts focused on extending these studies to other oxides5,
- Silica Force Fields Ersan Demiralp, T. Cagin, W. A. Goddard, III.
- Phase Transformations in Silica T. Cagin, Ersan Demiralp, W. A. Goddard, III.
- Simulation of Silica Glasses Ersan Demiralp, T. Cagin, W. A. Goddard, III.
- Modeling VPI-5 Osamu Kitao, Ersan Demiralp, T. Cagin, W. A. Goddard, III.
- in progress, Ersan Demiralp, T. Cagin, W. A. Goddard, III.
- in progress, Ersan Demiralp, Jan Sefcik, Osamu Kitao, T. Cagin, and W. A. Goddard, III
- in progress, Ersan Demiralp, Jan Sefcik, T. Cagin and W. A. Goddard, III.
Ersan Demiralp, Norman T. Huff, Jan Sefcik, Osamu Kitao and W. A. Goddard, III.
Last modified: December 1997.