Spin-coupled molecular orbitals: chemical intuition meets quantum chemistry

Molecular orbital theory is powerful both as a conceptual tool for understanding chemical bonding, and as a theoretical framework for ab initio quantum chemistry. Despite its undoubted success, MO theory has well documented shortcomings, most notably that it fails to correctly describe diradical states and homolytic bond fission. In this contribution, we introduce a generalised MO theory that includes spin-coupled radical states. We show through archetypical examples that when bonds break, the electronic state transitions between a small number of valence configurations, characterised by occupation of both delocalised molecular orbitals and spin-coupled localised orbitals. Our theory provides a model for chemical bonding that is both chemically intuitive and qualitatively accurate when combined with ab initio theory. Although exploitation of our theory presents significant challenges for classical computing, the predictable structure of spin-coupled states is ideally suited to algorithms that exploit quantum computers. Our approach provides a systematic route to overcoming the initial state overlap problem and unlocking the potential of quantum computational chemistry.