Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 2A / Section B / 23 / f

Endpoint energies, endpoint phase patterns and a conserved symmetry fix the qualitative connectivity. Each initial orbital must end in the appropriate product symmetry block, and the ordering within a generic block is preserved by avoided crossings. The two singleton blocks $A_2$ and $B_2$ have unique partners. No numerical calculation at each intermediate separation is required to draw those connections.

This conclusion is limited to a correlation diagram: it does not determine intermediate orbital energies, interaction strengths, the location or height of a reaction energy barrier, or the lowest route on a multidimensional potential energy surface. If the reaction breaks the assumed symmetry, previously distinct symmetry blocks can mix and the diagram must be reconsidered.