Calculation of intermolecular interaction energies by direct numerical integration over electron densities. 2. An improved polarization model and the evaluation of dispersion and repulsion energies

A procedure to adapt electron densities of isolated molecules for the evaluation of intermolecular energies, first introduced in paper 1 (Gavezzotti, A. J. Phys. Chem. B 2002, 106, 4145) is here improved for polarization energy and extended to dispersion and repulsion terms. Dispersion is evaluated from atomic polarizabilities distributed over the electron density, using an average ionization potential taken as the energy of the highest occupied molecular orbital, in a London-type inverse sixth-power formulation. Repulsion is evaluated from the overlap between electron densities. The method, called semiclassical density sums (SCDS), requires only four disposable numerical parameters and allows a complete evaluation of intermolecular interaction energies for a rather wide range of molecular systems. Calculations on molecular dimers, in comparison with results obtained by high-level quantum chemical methods, show that SCDS energies are quite reliable, at a fraction of the computational cost. The sublimation energies of many organic crystals are well reproduced by the corresponding calculated lattice energies, which include significant polarization contributions. The method allows a partitioning of intermolecular interactions over molecular pairs in crystals; this in turn allows for the first time a quantitative assessment of the relative importance of Coulombic, repulsion, and dispersion energies in the interaction between atom groups and in crystal packing in general, often contradicting some current views based on intuitive or even semiquantitative electrostatic models that do not include penetration terms.