The Block‐Localized Wavefunction (BLW) Perspective of Chemical Bonding

Abstract

The block-localized wavefunction (BLW) is the simplest variant of ab initio valence bond (VB) theory. It defines electron-localized (diabatic or resonance) states by restricting the expansion of molecular orbitals (MOs) within preset specific physical zones with coefficients optimized self-consistently. In contrast, conventional MO theory allows MOs to expand in the whole space of a system, thus provides a solution to electron-delocalized (adiabatic) states. Different definitions of electron localization schemes lead to different BLW's, corresponding to different diabatic (resonance) states. According to the classical resonance theory, the magnitude of electron delocalization is measured with the reference to the most stable resonance structure which now can be well defined with the BLW method. Consequently, the BLW method can be used to probe the energetic and geometric impacts from the electron delocalization, without needing other reference molecules. The applications of the BLW method are showcased here with the examples of the conjugation effect in benzene, the conjugation and hyperconjugation in planar and perpendicular butadiene and the hyperconjugation effect in ethane, as well as the energy decomposition analysis of the electron donor-acceptor (EDA) complex BH3NH3.