Valence-Bond Studies of AH2 Molecules. I. BeH2

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Abstract
The valence‐bond method is discussed and is compared with the molecular‐orbital SCF method from a computational standpoint. A procedure is outlined for performing valence‐bond calculations making use of the Prosser and Hagstrom biorthogonalization technique to evaluate the cofactors required when evaluating the energy using Löwdin's formula. A minimal basis set (STO) valence‐bond calculation on BeH2 including all possible configurations which do not involve excitation of the Be 1selectrons is reported. This is compared with a molecular‐orbital calculation using the same basis set. A valence‐bond wavefunction neglecting ionic configurations is shown to give an energy better than the molecular‐orbital wavefunction and almost as good as a ``full'' valence‐bond calculation. The perfect pairing and Craig and Thirunamachandran's resonance approximation are examined. A scheme for performing semiempirical valence‐bond calculations analogous to the Pople—Santry—Segal CNDO/2 MO method is described. Ionic configurations cannot be neglected in this ZDO approximation, but this is not a serious limitation. The method is very simple computationally. Hybrid and localized orbitals can be used.

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