Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2021 / Paper 2A / Section B / 24 / c / ii

No: the lowest dissociation channel is $\mathrm{He}+{\mathrm{H}}^{+}$, which a doubly occupied helium-centred orbital can describe. In the heteronuclear molecular orbital diagram the helium $1s$ site lies below the hydrogen $1s$ site. As separation grows, the bonding orbital tends to helium $1s$, the antibonding orbital tends to hydrogen $1s$, and the two ground-state electrons remain in the lower orbital.

There is therefore no symmetry requirement to keep both sites equally occupied and no compulsory half-weight mixture of wrong ionic configurations at dissociation. The physical asymptote is already a closed-shell atom plus a bare proton. The alternative ${\mathrm{He}}^{+}+\mathrm{H}$ limit lies higher. A Restricted Hartree-Fock method curve approaches the Hartree–Fock helium energy rather than the exact correlated helium energy, so ordinary dynamic electron correlation is still missing, but this is not the homonuclear static-correlation catastrophe. In the preceding two-panel sketch, the helium hydride curves have the same qualitative dissociation channel with a small residual correlation offset, whereas the hydrogen restricted curve reaches a qualitatively wrong high limit.