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.