G. Baskaran et V. Subrahmanyam, STABILITY OF MOLECULAR SINGLETS IN C-60 - A PARTIAL-SUMMATION APPROACH, Physical review. B, Condensed matter, 47(21), 1993, pp. 14611-14613
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.