Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 60 / c / ii

An example is the doubly occupied antibonding configuration $({\sigma }_u{)}^2$, rather than the bonding $({\sigma }_g{)}^2$ configuration dominant near equilibrium. It can be a self-consistent stationary determinant because symmetry makes the corresponding occupied-virtual Fock mixing vanish, while its bonding energy and nodal structure are inappropriate for the ground state. Solving the Hartree-Fock stationarity equations does not impose ground-state occupancy automatically.