A note on some valence bond calculations for the ground-state of the hydrogen molecule

The results of ab initio valence bond (VB) calculations are reported for H2, with up to 30 nuclear centred (15 at each H atom) 1s, 2s, 2p, 3s, 3p, 3d and 4s (cc-pVTZ+4s) atomic orbitals (AOs) included in them. A 30 AO complete active space VB calculation (CASVB) with 465 S = 0 spin structures gives an cc-pVTZ+4s energy of −1.17245 a.u. for d(H–H) = 1.4 a.u., which is identical to the CISD/cc-pVTZ+4s estimate obtained from a molecular orbital (MO) configuration interaction (CI) calculation with all single and double (CISD) substitutions.