Multiconfigurational SCF approach for high-symmetry molecules and its applications to fullerene trianion

Abstract

A symmetry-adapted multiconfiguration self-consistent field (MC SCF) approach aimed at calculations of high-symmetry molecules is proposed. The self-consistency procedure applicable to the molecular terms of any symmetry and multiplicity is developed. It holds the symmetry transformation properties of varied molecular orbitals, thus taking advantage of the relationships within the set of two-electron integrals through molecular invariants. For orbital optimization, a unified coupling operator is constructed on the basis of the pseudosecular method providing for efficient convergence to energy minimum. Based on the group-theory technique, computer codes have been developed for straightforward determination of the invariant expansions for two-electron integrals and configuration interaction (CI) matrix elements. Calculated in this way, the expansion coefficients are presented for the three-electron states that originate from joint t1u and t1g shells of an icosahedral fullerene C60, the case important for the calculations of anion C603− representing the charge state of the fullerene molecule in the superconducting ionic solids K3C60 or Rb3C60. The results of MC SCF calculations for lowest quasi-π-electronic states of C603− are discussed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 293–304, 1998