Limited Expansion of Diatomic Overlap (LEDO): A Near-Accurate Approximate Ab Initio LCAO MO Method. I. Theory and Preliminary Investigations

A method is presented for approximating multicenter integrals which arise in LCAO MO calculations utilizing Slater-type orbitals as basis functions. Two-center charge distributions are projected onto one-center functions, allowing all difficult three- and four-center integrals to be expressed as linear combinations of rapidly calculated one- and two-center Coulomb integrals. Test calculations are reported for a wide variety of polyatomic molecules yielding excellent agreement with corresponding ab initio treatments. Integral generation rates are well over an order of magnitude faster than current accurate approaches. The systematic and reliable nature of the approximation is examined and comparison made with the recently reported PDDO (projection of diatomic differential overlap) technique.