Absolutely local occupied and excited molecular orbitals in the third-order single excitation perturbation theory for...

The efficient code to evaluate the molecular interaction energy at the Hartree-Fock level of theory is developed. The method is the third-order single excitation perturbation theory based on the locally projected (LP) molecular orbitals (MO). It is shown that with a proper scaling of the molecular orbital integrals the basis set superposition error can be under controll even with a small size of basis sets. If the basis set contains augmented diffuse functions, the interaction energy, close to the counterpoise corrected energy, can be directly evaluated without the scaling, as fast as a single supermolecule Hartree-Fock calculation. The key of the method is to use the absolutely local occupied and excited MOs. With these MOs, the amount of the charge-transfer (CT) and the energy contribution of the CT terms are evaluated.