Long range C, N and H atom-atom potential parameters from<i>ab initio</i>dispersion energies for different azabenzene dimers

Abstract

After comparing theoretically the atom-atom model with the multipole expansion for the dispersion energy, we derive atom-atom potential parameters for C, N and H contacts by fitting the atom-atom dispersion energies to the dispersion interactions calculated by an ab initio method for the following (aza)-benzene molecules: benzene, pyridine, pyridazine, pyrimidine, pyrazine, s-triazine and s-tetrazine. The data base for the fit consists of 1200 randomly chosen configurations for each azabenzene dimer. The optimized parameters for the carbon and hydrogen contacts in particular are not unique ; reasonably good fits are obtained with different parameter sets. A very simple rule, which relates the atom-atom potentials to the isotropic molecular C6 dispersion coefficients, already leads to good estimates for the parameters. From the empirical parameter sets available in the literature the William-Govers set yields results which are close to our ab initio dispersion energies.