Self-consistent procedures for generalized valence bond wavefunctions. Applications H₃, BH, H₂O, C₂H₆, and O₂

Methods of efficiently optimizing the orbitals of generalized valence bond (GVB) wavefunctions are discussed and applied to LiH, BH, H₃, H ₂O, C₆H₆, and O₂. The strong orthogonality and perfect pairing restrictions are tested for the X ¹Σ⁺ state of LiH, the X ¹Σ ⁺, a ³∐, and A ¹∐ states of BH, and the H₂+D⇌ H+HD exchange reaction. The orbitals of H ₂O 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.