= Solution
\b[The fixed localized <determinant> is unsuitable near equilibrium.] It does not develop the correct shared bonding orbital and is not a pure singlet: for one alpha and one beta orbital, $\langle S^2\rangle/\hbar^2=1-|\langle A|B\rangle|^2$, which is nonzero unless the occupied <spatial orbitals> coincide. At dissociation it is an equal singlet/triplet mixture, though their energies become degenerate there. Near equilibrium the physical <ground state> is a bonding singlet. An optimized <UHF> calculation can recover the restricted, spin-pure solution by making its two occupied <spatial orbitals> equal; that does not rescue the particular fixed atomic-orbital <determinant> written here. A spin-adapted multideterminant description is needed to combine good bonding near equilibrium with correct singlet dissociation.
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