SPIN CONTAMINATION IN SINGLE-DETERMINANT WAVE-FUNCTIONS

Spin contamination in single-determinant wavefunctions is examined by performing unrestricted Hartree-Fock (UHF) calculations which constrain the <S triple-overdot 2> value using a Lagrange multiplier lambda. We call this method spin-constrained unrestricted Hartree-Fock (SUHF). It is shown that as lambda --> infinity, restricted open-shell Hartree-Fock (ROHF) solutions are obtained. Small values of lambda can significantly reduce the <S triple-over-dot 2> error, whilst changing the energy only slightly. Calculations are performed with SUHF and its Moller-Plesset counterpart SUMP2. SUMPn calculations with large values of lambda appear to be a way to carry out ROHF Moller-Plesset studies. Applications are presented for CN, NH2 and BH and are compared with UHF and annihilated UHF (AUHF) results.