The Coulson-Fischer + r₁₂ wavefunction for H₂

Abstract

The response of H₂ wavefunctions to the explicit incorporation of a correlating term linear in the inter-electronic coordinate (r ₁₂) is investigated for various simple molecular orbital and valence bond models. Particular attention is paid to the regions of space in which r ₁₂ tends to zero and so the wave-functions should exhibit a cusp. The role of ionic terms in the efficacy of the linear r ₁₂ term is analysed in some detail, as also is the variation with bond length (R) of the key adjustable parameters. The importance is emphasized of obtaining a wavefunction which exhibits qualitatively correct behaviour as a function of R before the inclusion of explicit r ₁₂ dependence.