Dynamic polarizability, dispersion coefficient C-6 and dispersion energy in the effective fragment potential method

The development of a fragment-fragment dispersion energy expression, for the general effective fragment potential (EFP2) method is presented. C-6 dispersion coefficients, expressed in terms of the dynamic polarizabilties over the imaginary frequency range (alpha(inu)), were calculated for a set of homo and hetero dimers. Using these coefficients the dispersion energy has been calculated. The dispersion energy is expressed using a simple London series expansion terminated after the n = 6 term and implemented using distributed localized molecular orbitals (LMOs). The EFP2 dispersion energy is compared to symmetry adapted perturbation theory (SAPT) values. From this comparison, it is apparent that one needs to include higher order terms in the dispersion energy. Adding an estimated C-8 term to the C-6 energy greatly improves the agreement with the benchmark SAPT energies.