Evidence From Previous OBS-GMCSC Work

For instance, GMCSC calculations on the boron anion [2] and on the dilithium molecule [26], both in their ground states, have shown how singleconfiguration wavefunctions, including spin-coupled ones, can be hard-put to provide a robust description of certain highly-symmetric systems. By robust description , we mean one that will not change, at least qualitatively, as more configurations are added to the wavefunction. 

For B (X3P), the fully symmetric configuration was the obvious (si s2 s3 s4 px py), with si and s2 a pair of strongly overlapping inner orbitals, s3 and s4 more difftise, but nodeless, valence s orbitals, and (px, py) a pair of symmetry-equivalent 2p orbitals. The two symmetry-related configurations were (s i S2 s3 

Similarly for Li2 (X Eg ) the two symmetry-related configurations, which, like the fully symmetric one, included a orbitals only, were connected by inversion through the molecular centre. The three configurations, far from being orthogonal, had large mutual overlaps. 

In both cases, the extra pair of configurations entered the multiconfiguration wavefunction with a weight that was comparable to that assumed in it by the fully symmetric configuration. Here and in what follows, a configuration s weight is defined as its Chirgwin-Coulson occupation number [3 4] [2] (for alternative definitions, see e.g. Refs. [35] and [36]). 

Obviously, both the fully symmetric and the pair of symmetry-related configurations had to be viewed as reference configurations, and taken into account in qualitative discussions of the electronic structure. 

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