Spectroscopic constants and bonding features of the low‐lying states of LiB and LiB + : Comparative study of VBSCF and MO theory

Abstract The common correspondence between molecular orbital theory and resonance theory in the description of the electronic structure of a molecule is used to select valence bond (VB) structures constructing wave functions of the low‐lying states of LiB and LiB + . The spectroscopic parameters of the low‐lying states of LiB and LiB are determined by using the valence bond self‐consistent field (VBSCF) method. For comparison, multconfiguration SCF (MCSCF) calculations for LiB are also carried out. If the overlap‐enhanced orbitals are employed, a compact VB wave function can correctly describe bond making and bond breaking in the entire dissociation process for the low‐lying electronic states of LiB. All calculations locate the ground state as 3 Π. The VB calculation with 14 VB structures at the level of the basis set DH( s ,  p ) predicts an excellent dissociation energy of 1.16 eV and vibrational frequency of 527 cm −1 for the ground state, which is in good agreement with previous high‐level calculations with a large basis set. The effect of the basis set on the numerical quality of the VBSCF calculation is investigated. It is important for improving accuracy of the VB calculation to enlarge the basis set, although the VB treatment including more VB structures with a relatively small basis set needed in the nonorthogonal VB calculation can reasonably describe dissociation behavior for systems with few electrons. The chemical bonds in the ground state 3 Π and the excited state (1) 3 Σ − have ionicities of 63.4 and 65.1%, respectively, while chemical bond in the first excited state 1 Σ + is mainly covalent. Other several low‐lying states are also investigated by the VB and MCSCF methods. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 283–290, 1998