SPECTROSCOPIC CONSTANTS AND BONDING FEATURES OF THE LOW-LYING STATES OF LIB AND LIB- COMPARATIVE-STUDY OF VBSCF AND MO THEORY( )

The common correspondence between molecular orbital theory and resonan ce theory in the description of the electronic structure of a molecule is used to select valence bond (VB) structures constructing wave func tions of the low-lying states of LiB and LiB+. The spectroscopic param eters of the low-lying states of LiB and LiB are determined by using t he valence bond self-consistent field (VBSCF) method. For comparison, multconfiguration SCF (MCSCF) calculations for LiB are also carried ou t. If the overlap-enhanced orbitals are employed, a compact VB wave fu nction can correctly describe bond making and bond breaking in the ent ire dissociation process for the low-lying electronic states of LiB. A ll calculations locate the ground state as (3)Pi. The VB calculation w ith 14 VB structures at the level of the basis set DH(s, p) predicts a n excellent dissociation energy of 1.16 eV and vibrational frequency o f 527 cm(-1) for the ground state, which is in good agreement with pre vious high-level calculations with a large basis set. The effect of th e basis set on the numerical quality of the VBSCF calculation is inves tigated. 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 nonorthogon al VB calculation can reasonably describe dissociation behavior for sy stems with few electrons. The chemical bonds in the ground state (3)Pi and the excited state (1)(3)Sigma(-) have ionicities of 63.4 and 65.1 %, respectively, while chemical bond in the first excited state (1)Sig ma(+) is mainly covalent. Other several low-lying states are also inve stigated by the VB and MCSCF methods. (C) 1998 John Wiley & Sons, Inc.