Damping functions in the effective fragment potential method

This work presents the implementation and analysis of several damping functions for Coulomb, induction, and dispersion interactions within the framework of the general effective fragment potential (EFP) method. Damping is necessary to obtain the correct asymptotic short-range behavior of these interactions. Correctly chosen damping functions allow a balanced description of different parts of intermolecular potential energy surfaces and improve the accuracy of predicted intermolecular distances and binding energies. The performance of different damping functions is tested by comparing the EFP energy terms with the symmetry adapted perturbation theory (SAPT) energy terms in a range of intermolecular separations for ten molecular dimers. The total EFP binding energies in these dimers were compared with the binding energies obtained from SAPT and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. A formula for electrostatic damping that was derived from first principles is recommended. This method employs the overlap of fragment localized molecular orbitals (LMO) within the spherical Gaussian approximation. The LMO overlap integrals are also used to determine the damping of dispersion. Gaussian polarization damping functions are recommended for use within the EFP framework. With this set of damping functions, the EFP binding energies are within 0.5 kcal/mol and intermolecular equilibrium separations are within 0.2 angstrom of the corresponding CCSD(T) and SAPT values. This consistent accuracy of EFP is encouraging for future studies of more complicated molecular complexes.