The decomposition of intermolecular interaction energies in localized orbitals — critical analysis and an invariance

For the straight-forward decomposition of the correlation contributions to intermolecular interaction energies in localized orbitals, the dependence on orbital localization is investigated. It is found that from the six classes of excitations intra-molecular contributions, as well as one ionic configuration and the basis set superposition error are very sensitive to the localization in the virtual orbital space, whereas the other three terms are less affected. An invariance of the individual contributions with respect to orbital rotations within the monomer spaces is formulated, which restricts the dependence of results on arbitrary orbital rotations and permits significant speed-ups, at least in Møller–Plesset second-order perturbation theory.