A systematic classification of atomic integrals in a b i n i t i o molecular orbital calculations. I. The core valence approximation molecular orbital method

A computational scheme for the systematic approximation of certain two-electron integrals in ab initio molecular orbital calculations is described in which core–valence overlap distributions are expanded in terms of monocentric functions centered at the core. The integral evaluation time now varies as (Nvalence)4 rather than (Ntotal)4, thus resulting in substantial saving of computer time. The method is shown to be invariant to unitary transformations of the basis set and to be capable of yielding wave functions to any required accuracy. For most purposes a level of approximation denoted B1 is found to be adequate, yielding molecular energies accurate to 10−3 a.u. with no appreciable error in other molecular properties.