An Assessment of Molecular Dynamic Force Fields for Silica for Use in Simulating Laser Damage Mitigation Page: 3 of 11
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This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-
An Assessment of Molecular Dynamic Force Fields for Silica for Use in
Simulating Laser Damage Mitigation
Thomas F. Soules, George H. Gilmer, Manyalibo J. Matthews, James S. Stolken and Michael D. Feit
National Ignition Facility and Photon Sciences, Lawrence Livermore National Laboratory
7000 East Avenue, Livermore, California 94550
We compare force fields (FF's) that have been used in molecular dynamic (MD) simulations of silica in
order to assess their applicability for use in simulating IR-laser damage mitigation. Although pairwise
FF's obtained by fitting quantum mechanical calculations such as the BKS and CHIK potentials have
been shown to reproduce many of the properties of silica including the stability of silica polymorphs and
the densification of the liquid, we show that melting temperatures and fictive temperatures are much too
high. Softer empirical force fields give liquid and glass properties at experimental temperatures but may
not predict all properties important to laser mitigation experiments.
Keywords: Molecular dynamics, silica structure, silica glass properties, laser damage mitigation.
CO2 laser treatments are a common method of mitigating surface damage sites generated on silica optics
by the high fluence laser beams. Depending on the pulse duration and intensity, this process can involve
flow and densification and/or evaporation or even ablation. During the CO2 laser pulse the damaged
silica region is heated to between 2000 and 5000 K in 10-6 to 1012 s. The effects of these laser pulses are
being modeled with finite-element tools. However basic properties of silica such as heat capacities,
thermal conductivities, thermal expansion and densification, and fictive temperatures under these
conditions of very rapid heating and cooling to very high temperatures are often unknown and cannot be
determined by conventional experimental methods. Molecular dynamics (MD) has been shown to predict
some of the properties of fused silica including atomic structure pair correlation functions, the stability of
silica polymorphs and densification. An MD model with realistic force-fields (FF's) may provide these
quantities for use in predictive finite-element tools. Also direct MD simulation of the laser mitigation
experiment may also be possible and can suggest improvements to the laser mitigation protocol.
The purpose of this paper is to compare simple radial force-fields (FF's) previously used in MD
simulations of silica in order to assess their ability to predict the properties of silica that are important in
laser damage mitigation by means of a CO2 laser. Four Si-O pairwise FF's are shown in Figure 1 as
function of radial distance, r, between the atoms along with the analytical expressions for all the distinct
pairs: Si-O, 0-0 and Si-Si. There is a large difference in the Si-O bonding energies from these FF's. The
two "strong" fields, BKS and CHIK 2 were obtained by fitting a Hartree-Fock quantum mechanical
calculation and fitting the structure obtained from a density functional Car-Parinello calculation
respectively. The two "softer" potentials are empirical with the Takada3 force field being the sum of a
Morse potential plus Coulomb interactions, and the Soules FF being an ionic Born-Mayer potential with
charges scaled to give a reasonable melting temperature4.
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Soules, T F; Gilmer, G H; Matthews, M J; Stolken, J S & Feit, M D. An Assessment of Molecular Dynamic Force Fields for Silica for Use in Simulating Laser Damage Mitigation, article, October 21, 2010; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc834319/m1/3/: accessed January 22, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.