The single excitation perturbation expansion theory based on the locally projected molecular orbitals for molecular interaction: Comparison with the counterpoise corrected energy

The single excitation second order perturbation expansion (SPT) based on the locally projected self-consistent field molecular orbitals for molecular interaction is numerically tested for isomers of uracil–water and for 24 water clusters. For the extensive basis sets, the calculated binding energy BESPT becomes close to the counterpoise corrected binding energy BE SCF CP . The required cpu time is less than that for a single supermolecule calculation. For smaller basis sets, by restricting the excited orbitals to the strictly monomer basis set (SMBS), BESPT becomes approximate to BE SCF CP . The use of the SMBS for the excited orbitals has a clear advantage over the orthogonal but delocalized excited orbitals.