Efficient computer modeling of organic materials. The atom-atom, Coulomb-London-Pauli (AA-CLP) model for intermolecular electrostatic-polarization, dispersion and repulsion energies

An atom-atom intermolecular force field with subdivision of interaction energies into Coulombic-polarization, dispersion (London) and repulsion (Pauli) terms is presented. Instead of using fixed interaction functions for atomic species, atom-atom potential functions are calculated for each different molecule on the basis of a few standard atomic parameters like atomic numbers, atomic polarizability and ionization potentials, and of local atomic point charges from Mulliken population analysis. The energy partitioning is conducted under guidance from the more accurate evaluation of the same terms by the PIXEL method, also highlighting some intrinsic deficiencies of all atom-atom schemes due to the neglect of penetration energies in Coulombic terms on localized charges. The potential energy scheme is optimized for H, C, N, O, Cl atoms in all chemical connectivities and can be extended to F, S, P, Br, I atoms with minor modifications. The scheme is shown to reproduce the sublimation heats of 154 organic crystal structures, to reproduce about 400 observed crystal structures without distortion, and to reproduce heats of evaporation and specific gravities of 12 common organic liquids. It is therefore suitable for both static and evolutionary (Monte Carlo) molecular simulation. Fine tuning of the four terms for specific systems can be easily performed on the basis of chemical intuition, by the introduction of one overall damping factor for each of them. The scheme is embedded in a suite of Fortran computer programs portable on any platform. For reproducibility and general use, source codes are available for distribution.