Local quantum chemistry. Incorporation of pseudospectral methodology into the local space approximation

We show that introducing pseudospectral (PS) methodology into local space approximation (LSA) treatments reduces the scaling with system size by a factor of N, N being a measure of size, at both Hartree-Fock and correlated levels. Efficient ways to organize Hartree-Fock calculations, avoiding storage of full and local space two-electron integrals, are presented. A similar analysis applies if the PS approach is replaced by so-called resolution of the identity integral evaluation. The possibility of major savings of computer time, unique to LSA calculations and due to the rapid fall-off of density matrix elements with interatomic distance, is discussed.