THE COULSON-FISCHER PLUS R(12) WAVE-FUNCTION FOR H2

The response of H-2 wavefunctions to the explicit incorporation of a correlating term linear in the inter-electronic coordinate (r12) is investigated for various simple molecular orbital and valence bond models. Particular attention is paid to the regions of space in which r12 tends to zero and so the wave-functions should exhibit a cusp. The role of ionic terms in the efficacy of the linear r12 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 r12 dependence.