Anthony C. Hess

Ab initio valence force field calculations for quartz

We have derived valence force constants for the tetrahedral SiO4 unit and the inter-tetrahedral SiOSi linkage from previous ab initio molecular orbital calculations on H4SiO4 and H6Si2O7 using a split-valence polarized Gaussian basis set (6-31G*), and used these to calculate the infrared and Raman active vibrational modes of α-quartz. The calculation gives frequencies approximately 15% greater than experiment, as expected from harmonic force constants obtained at this level of Hartree-Fock theory, but the calculation gives the correct distribution of modes within each frequency range. Calculated 28–30 Si and 16–18 O isotope shifts and pressure shifts to 6 GPa are also in reasonable agreement with experiment. We have also used our ab initio force field to calculate the vibrational spectrum for β-quartz. The results suggest either that inclusion of a torsional force constant is important for determining the stability of this high temperature polymorph, or that the β-quartz has a disordered structure with lower symmetry (P62) domains, as suggested by earlier diffraction studies.

A valence force field for diamond from ab initio molecular orbital cluster calculations

The lattice dynamics and elastic moduli of diamond are reinvestigated using a method based upon an ab initio valence force field obtained for the neopentane molecule. The calculated phonon dispersion relations are in very good agreement with experiment particularly with respect to the LA, LO, TO vibrational modes. The results demonstrate the transferability of force fields from ab initio calculations on suitably chosen molecular clusters to condensed phases, and also show that a five-parameter valence force field model is sufficient to reproduce most of the features of the lattice dynamics of diamond. Excellent agreement was found between calculated and experimental bulk moduli indicating adequate modelling of the bond stretching interactions. Deviations of the transverse acoustical vibration modes at low frequencies and the remaining elastic moduli are associated with bond angle interaction terms, which may be due to the omission of a longer range interaction force constant.

Structural and vibrational properties of neopentane and tetramethylsilane using ab initio mo calculations

Abstract The geometry and vibrational properties of neopentane and tetramethylsilane have been determined from ab initio calculations. The 6–31 G* basis level geometries along with analytically determined frequencies and force constants are presented and compared with experiment. A complete and non-redundant set of internal symmetry coordinates is presented and used for the normal coordinate treatment of the molecular force constants. Both molecules are determined to have T d point symmetry. The torsional A 2 and F 1 frequencies which are inactive in both infrared and Raman are calculated as 221 and 302 cm −1 for neopentane and 144 and 168 cm −1 for tetramethylsilane respectively.