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 antisymmetrized product) is restored by spin projection. Full variation after projection is examined for two models. One is the long known spin-projected, extended Hartree-Fock (EHF). The other is a yet unexplored function, termed spin-projected, extended antisymmetrized product of strongly orthogonal geminals (EAPSG). Studies on size consistency are presented for both models. Numerical evaluation of EHF and EAPSG is performed for small test systems (H4 and H8).