Many-Electron Wavefunctions Expanded in Spin-Adapted Antisymmetrized Products, and Their Expectation Values

Abstract

Unrestricted configuration‐interaction wavefunctions are constructed according to certain conventions which simplify the calculation of expectation values when S is a good quantum number. General wavefunctions are expressed in terms of antisymmetric space‐spin functions that are eigenstates of S² and S_z and form a complete orthonormal set. The building blocks, called ``spin‐adapted antisymmetrized products'' or SAAP's, are designed to take advantage of double occupancy. It is shown that SAAP's, when constructed from orthonormal space orbitals, can be handled in multiconfigurational calculations more easily than Slater determinants. Simple formulas are derived for matrix elements of observable operators expressible in terms of one‐ and two‐electron integrals.