Development of an improved Stillinger-Weber potential for tetrahedral carbon using ab initio (Hartree-Fock and MP2) methods

An improved interatomic potential for tetrahedral carbon is presented. This potential is of the Stillinger-Weber (SW) type and has been determined from calculations performed on a select group of small hydrocarbon molecules, chosen for their similarities to the tetrahedral lattice of bulk diamond. Counterpoise corrected Hartree-Fock (HF) and second-order Møller-Plesset perturbation theory (MP2) calculations were performed on ethane, 2,2-dimethylpropane (neopentane, (C₅H₁₂), 2-dimethyl-3-dimethylbutane (neobutane, C₈H₁₈) and cyclohexane (C₆H₁₂) in order to determine the two-body (stretching) and three-body (bond bending) energies. The suitability of these molecules to model the properties of diamond was determined by comparison of CC bond length, well depth, CCC bond angle, simultaneous stretch and bend energy and force constants to those of bulk diamond. It was found that neopentane provided the best overall description of tetrahedral bonded carbon. The ab initio derived stretch and bend energies were fitted to the SW potential energy terms and the SW parameters calculated. The newly parametrized SW potential was then evaluated by calculating the stretch force constants, elastic constants and the X-point phonon modes of bulk diamond.