Self-Consistent Procedures for Generalized Valence Bond Wavefunctions. Applications H3, BH, H2O, C2H6, and O2

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Abstract
Methods of efficiently optimizing the orbitals of generalized valence bond (GVB) wavefunctions are discussed and applied to LiH, BH, H3, H2O, C6H6, and O2. The strong orthogonality and perfect pairing restrictions are tested for the X 1Σ+ state of LiH, the X 1Σ+, a 3π, and A 1π states of BH, and the H2+D⇄H+HD exchange reaction. The orbitals of H2O and C2H6 naturally localize into OH, CH, and CC bonding pairs. The nonbonding orbitals of H2O are approximately tetrahedral but this description is only 2 kcal lower than the optimum description in terms of σ and π lone-pair functions. The calculated rotational barrier for C2H6 is 3.1 kcal, in good agreement with the experimental value (2.9 kcal). The description of the O2 molecule in the GVB approach is presented and the results of carrying out CI calculations using the GVB orbitals are discussed. The GVB orbitals are found to be a good basis set for configuration interaction calculations. The general features of GVB orbitals in other molecules are summarized.