Ab Initio Valence Bond Theory for Strongly Correlated Systems.

Strongly correlated systems, characterized by significant multiconfigurational character, pose a persistent challenge in quantum chemistry. While molecular orbital (MO)-based multiconfigurational self-consistent field methods such as CASSCF and CASPT2 have become standard tools for treating such systems, valence bond (VB) theory offers a conceptually distinct and chemically intuitive alternative. Rooted in the classical Lewis structure framework, VB theory provides a compact and localized description of electron pairing, making it especially well-suited for strongly correlated systems. This review presents a comprehensive overview of the methodological development and practical applications of ab initio VB approaches, including VB self-consistent field (VBSCF), breathing orbital VB (BOVB), VB configuration interaction (VBCI), VB perturbation theory (VBPT2), and density functional VB (DFVB) methods. Particularly, the VBPT2 and DFVB methods enable accurate treatment of bond dissociation, excitation energies, and reaction barriers. Benchmark comparisons demonstrate that VB-based methods achieve performance comparable to established MO-based methods. The findings highlight the promise of VB theory as a powerful and interpretable framework for advancing the theoretical understanding of strongly correlated systems.