STABILITY OF MOLECULAR SINGLETS IN C-60 - A PARTIAL-SUMMATION APPROACH

The hypothesis of the stability of molecular singlet states that in la rge carbon (p - pi)-bonded systems the collective effect of the intram olecular repulsion can stabilize the low-spin state, as opposed to a h igh-spin state (suggested by Hund's rule). We provide further support to this hypothesis by improving an existing degenerate-perturbation th eory into an infinite partial summation in a simple Hubbard-model syst em for C-60(2). The critical U/t at which the singlet becomes stable i s U/t approximately 2.19 in our calculation, as opposed to the second- order perturbation-theory result of U/t approximately 3.