Development of potential energy parameters for the modeling of condensed-phase silicates is relying increasingly on the results of a b initio calculations. We examined the effects of basis set size and electron correlation corrections on the properties of three increasingly complex molecules often used as models for silicates, disiloxane (H3SiOSiH3), orthosilicic acid and disilicic acid (H6Si207). When disiloxane was optimized with the MC6-311G(2d,2p) basis, MP2 corrections changed the HF results through narrowing the Si-0-Si angle by 8" and lengthening each Si-0 bond by 0.022 A. When the same basis was applied to Si(OH)4, the MP2-optimized bond lengths for Si-0 and 0 -H were 0.020 and 0.017 A larger, respectively, than their HF-optimized values. Meanwhile, the Si-0-H angle decreased 3" from its HF value, to 115.4' at the MP2 level. In comparing these results to the HF/ and MP2/MC6-311G** optima, it was evident that the basis set limit had not been reached. Due to the hydrogen-bonding interactions in H6Si207, the MP2/ MC6-3 l l G * * correlation corrections to geometries of the three conformers considered here were more dramatic. The Si-0 bond lengths increased by as much as 0.029 A over their HF/MC6-311G**-optimized values, 0 -H bonds lengthened by up to 0.021 A, and Si-0-Si angles decreased by up to 13.7'. Some framework Si-0-Si-0 torsions changed by 10" when electron correlation effects were included in the optimization, and nonbonded OH distances decreased by as much as 0.62 A. Further optimizations showed that only two of the conformers were stable at the MP2 level, emphasizing the importance of electron correlation in the study of silicate molecular models. The two MPZoptimized structures of H6Si207 were separated by 2.4 kcal mol-' , but when zero-point energies and thermal corrections were included, the conformers were within 0.4 kcal mol-'. It is argued on the basis of electrostatic potential maps and a b initio partial charges that H3SiOSiH3 and its analogues provide poor quantitative models for silicate functional groups. However, structural features and partial charges on Si(OH)4 and H6Si207 proved to be very similar at a given level of theory. Thus, effective a b initio parameter development for silicates should focus on molecules comprised of silicon that is fully coordinated by oxygen.
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