Empirical intermolecular potentials for organic crystals: the `6-exp' approximation revisited

Empirical intermolecular potentials have been developed for organic crystals containing C, H, N, O, Cl and S atoms without hydrogen bonds, using atom-atom radial functions in the form E=Aexp(-BR)-CR -6 . The number of disposable parameters is thus reduced to a minimum, there is no need to evaluate atomic or site charges, and convergence problems connected with the use of Coulombic R -1 terms are avoided. The optimization of the potential parameters A, B, C for each pair of atomic species has been carried out using as observables the distribution of atom-atom distances in 1846 organic crystals, the heats of sublimation of 122 compounds, and the structural data (rigid-body positional parameters and cell dimensions) for 217 crystals