Valence bond configuration interaction: A practical ab initio valence bond method that incorporates dynamic correlation

A post-VBSCF method, called valence bond configuration interaction (VBCI), is developed here. The method incorporates dynamic correlation, by use of configuration interaction (CI) to improve the energetics after a VBSCF calculation. The levels of CI are fashioned as in the corresponding molecular orbital approach. Thus, VBCIS involves only single excitations, while VBCISD involves also doubles, and so on. The VBCI method retains the simplicity of a VB presentation by expressing the wave function in terms of a minimal number of effective structures that dictate the chemistry of the problem. The method was tested by calculating the bond energies of H₂, LiH, HF, HCl, F₂, and Cl₂ as well as the barriers of identity hydrogen abstraction reactions, X• + X′H → XH + X′• (X, X′ = CH₃, SiH₃, GeH₃, SnH₃, or PbH₃). It is shown that VBCIS gives results that are at par with breathing orbital valence bond method. The VBCISD method is better and its results match those of the molecular orbital based coupled cluster CCSD method. Future potential directions of the development of the VBCI approach are outlined.