ABSTRACT

The Hamiltonian used in most molecular calculations does not contain spin-dependent terms and so the expectation values of the observables depend only on the spatial functions. In most of the conventional treatments the wavefunction is a linear combination of antisymmetrized products of spatial and spin functions. The spin eigenfunctions also generate a representation of the symmetric group, these are associated with Young tableaux with at most two rows and these representations are conjugate to the representations generated by the spatial functions. The spin-coupled wavefunction is based on one spatial orbital and the best combination of the all the possible spin functions. The nonorthogonal configuration interaction calculation based on the spin-coupled valence bond functions is carried out using a cofactor-driven program which is based on the Lowdin formulation of matrix elements of the Hamiltonian between Slater determinants composed of nonorthogonal orbitals.