Spin Symmetry and Size Consistency of Strongly Orthogonal Geminals

An overview of geminal-based Wavefunctions is given, allOwing for singlet triplet mixing within the two-electron units. Spin contamination of the total wavefunction (obtained as an antisynametrized product) is restored by spin projection. Full variation after projection is examined for two models. One is-the long known spinprojected, extended Hartree Fock (EHF). The other is a yet unexplored function, termed spin-projected, extended antisymmetrized product of strongly orthogonal geniinals (EAPSG). Studies on size consistency are presented for both models. Numerical evaluation of EHF and F_APSO is performed for small test systems (H-4 and H-8).