Ab initio effective potentials for use in molecular quantum mechanics

We have investigated the method of effective potentials for replacing the core electrons in molecular calculations. The effective potential has been formulated in a way which simplifies computations while producing wave functions of ab initio quality. The effective potential is expressed in an analytic form which (i) represents the actual ab initio nonlocal potential (as defined by the matrix elements for a given basis set) and (ii) permits efficient computations of the effective-potential integrals (by incorporating the properties of Gaussian basis functions). To minimize the number of basis functions required in the molecular calculations, we define a new ab initio effective potential derived from modified Hartree-Fock valence orbitals whose core character has been removed. The effective-potential method as formulated becomes a very strong and reliable tool in attempting calculations on very large molecules. Applications to Li, Na, and K are presented.