GENERALIZED VALENCE-BOND MOLECULAR EQUILIBRIUM GEOMETRIES

Equilibrium geometries have been obtained at the generalized valence b ond (GVB) level for one- and two-heavy-atom molecules and radicals (AH (n) and AH(m)BH(n)) containing first- or second-row elements. The resu lts are compared with the available experimental and theoretical (HF, MP2 and CID) values. The effect of basis set on the GVB equilibrium ge ometries is also discussed. The results indicate that the addition of d-polarization or diffuse functions to the split valence basis set is necessary to obtain reasonable GVB geometries. In general, geometrical parameters from the GVB/6-31G calculations, which treat electron cor relation by using the coupling of GVB perfect-pairing orbitals, are in better agreement with the experimental data than those obtained from the corresponding HF wavefunction. The GVB/6-31G geometries are close to the corresponding MP2 or CID geometries. GVB calculations are espe cially useful for the proper description of multiple bonds.