Choice of Computational Techniques and Molecular-Models for Ab-Initio Calculations Pertaining to Solid Silicates

by Teppen, B. J.; Miller, D. M.; Newton, S. Q.; Schafer, L.

Development of potential energy parameters for the modeling of condensed-phase silicates is relying increasingly on the results of ab 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 (Si(OH)(4)), and disilicic acid (H6Si2O7) When disiloxane was optimized with the MC6-311G(2d,2p) basis, MP2 corrections changed the HF results through narrowing the Si-O-Si angle by 8 degrees and lengthening each Si-O bond by 0.022 Angstrom. When the same basis was applied to Si(OH)(4), the MP2-optimized bond lengths for Si-O and O-H were 0.020 and 0.017 Angstrom, larger, respectively, than their HF-optimized values. Meanwhile, the Si-O-H angle decreased 3 degrees from its HF value, to 115.4 degrees 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 H6Si2O7, the MP2/MC6-311G** correlation corrections to geometries of the three conformers considered here were more dramatic. The Si-O bond lengths increased by as much as 0.029 Angstrom over their HF/MC6-311G**-optimized values, O-H bonds lengthened by up to 0.021 Angstrom, and Si-O-Si angles decreased by up to 13.7 degrees. Some framework Si-O-Si-O torsions changed by 10 degrees when electron correlation effects were included in the optimization, and nonbonded OH distances decreased by as much as 0.62 Angstrom. 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 MP2-optimized structures of H6Si2O7 were separated by 2.4 kcal mol(-1), but when zero-point energies and thermal corrections were included, the conformers were within 0.4 kcal mol(-1). It is argued on the basis of electrostatic potential maps and ab 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 H6Si2O7 proved to be very similar at a given level of theory. Thus, effective ab initio parameter development for silicates should focus on molecules comprised of silicon that is fully coordinated by oxygen.

Journal of Physical Chemistry
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