Electronic correlation contribution to the intermolecular interaction energy from symmetrized systematic molecular fragmentation model

The applicability of the systematic molecular fragmentation (SMF) model to reproduce the correlation part of intermolecular interaction energies has been studied for a new variant of the SMF, denoted by us as symmetrized SMF (SSMF). At third level of fragmentation (SSMF3) this contribution is reproduced with a small error of a few per cent in comparison to the accurate value. We have also examined the Hartree-Fock part of the interaction energy and found that for large intermolecular distances it is only qualitatively correct. However, the calculation of the Hartree-Fock interaction energies can be easily performed even for large molecules, while the correlated part of the interaction energy can be obtained with a much higher computational effort. Therefore, we propose to evaluate the interaction energy with a hybrid method, where the uncorrelated part is obtained in a classical way, while for the expensive electron-correlated part the SSMF approach is applied.