Application of quantum chemistry to geochemistry and geophysics

Abstract

Silicates are a class of systems for which the use of quantum chemistry has been minimal with respect to their importance in earth sciences as well as in different parts of chemistry such as catalysis and material chemistry. The determination of reliable molecular mechanics parameters for such systems is a necessary starting point for molecular dynamics simulations of their liquid phase in order to derive estimates of the thermodynamic parameters over a wide range of pressure and temperature. The different quantum chemical approaches that are able to provide site-site potentials, namely prototype molecule or cluster semi-empirical or ab initio calculations, the Car-Parrinello method and the periodic Hartree-Fock method, are critically reviewed. The periodic Hartree-Fock method appears to be a very convenient tool for such studies. This is demonstrated by the results obtained on the silica polymorphs for which accurate geometries are calculated even with minimal basis sets. Phase transitions have been investigated for SiO 2, GeO 2 and MgO. The usefulness of core Pseudopotentials is illustrated by recent calculations on rather large unit cell systems such as the spinel phase of Mg 2SiO 4 and on the polymorphs of Al 2SiO 5. The performances of different types of basis set are discussed in order to provide choice criteria.