SPIN-FREE APPROACH FOR EVALUATION OF ELECTRONIC MATRIX-ELEMENTS USINGCHARACTER OPERATORS OF L(N)

A spin-free method is presented for evaluating electronic matrix eleme nts over a spin-independent many-electron Hamiltonian. The spin-adapte d basis of configuration state functions is obtained using a nonorthog onal spin basis consisting of projected spin eigenfunctions. The gener al expressions for the matrix elements are given explicitly, and it is demonstrated how the matrix elements may be obtained simply from the knowledge of the irreducible characters of the permutation group L(N). The presented formulas are very general and may be applied in connect ion with both spin-coupled valence bond studies and in conventional co nfiguration interaction (CI) methods based on an orthonormal orbital b asis. (C) 1996 John Wiley & Sons, Inc.