Electron Correlation and Separated Pair Approximation in Diatomic Molecules. I. Theory

The theory of antisymmetrized products of separated geminals for many-electron systems is developed for arbitrary spin states. The geminals are expanded in terms of their natural spin orbitals, and methods are developed for their optimal determination. New variational equations are derived and cast in the form of one pseudoeigenvalue equation for all natural spin orbitals. An alternative scheme is given for finding optimal symmetry-adapted natural orbitals in terms of an orthonormalized basis set by direct energy minimization. A strategy for optimization is discussed which was found useful in calculations of the ground states of the molecules LiH, BH, NH, and their separated atoms.