  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.
