Abstract
In order to test the effect of a removal of an “equivalence” restriction, some excited states of B, C^+^ and C arising from the configuration 1__s__^2^2__s__2__p__^n^ (n = 2, 3) are calculated by the use of a one‐diffuse‐electron model with the optimal spin coupling (DO), in which the freedom of the spin coupling is fully used. The configuration employed is 1__s__^2^2__s__2__p__^n–1^2__p__′ (n = 2, 3).
The DO method gives a good agreement with the term energies especially for boron (^4^P → ^2^P) and carbon (^5^S° → ^3^S°). A diffuse 3__p__ natural orbital with a large occupation number is found for boron ^2^P and carbon ^3^S° by DO. This result is confirmed by configuration interaction calculations.
Using the same configuration as above, we also examine a one‐diffuse‐electron model with a fixed spin coupling for boron. This method almost reproduces the results of restricted Hartree–Fock and the effect of a removal of an “equivalence” restriction is not found.
The importance of utilizing the freedom of the spin coupling in a one‐diffuse‐electron model is strongly indicated in this article.