Second-order energy components in basis-set-superposition-error-free intermolecular perturbation theory

Recently a basis-set-superposition-error-free second-order perturbation theory was introduced based on the "chemical Hamiltonian approach" providing the full antisymmetry of all wave functions by using second quantization. Subsequently, the "Heitler-London" interaction energy corresponding to the sum of the zero- and first-order perturbational energy terms was decomposed into different physically meaningful components, like electrostatics, exchange and overlap effects. The first-order wave function obtained in the framework of this perturbation theory also consists of terms having clear physical significance: intramolecular correlation, polarization, charge transfer, dispersion and combined polarization-charge transfer excitations. The second-order energy, however, does not represent a simple sum of the respective contributions, owing to the intermolecular overlap. Here we propose an approximate energy decomposition scheme by defining some "partial Hylleraas functionals" corresponding to the different physically meaningful terms of the first-order wave functions. The sample calculations show that at large and intermediate intermolecular distances the total second-order intermolecular interaction energy contribution is practically equal to the sum of these "physical" terms, while at shorter distances the overlap-caused interferences become of increasing importance.