Ground and excited Hubbard states for buckminsterfullerene with uniform and alternating bond strengths

A detailed investigation on the pi-electronic ground and lowest excited g- and u-symmetrical singlet and triplet states of icosahedral C-60 is performed in the framework of the semiempirical Hubbard model using single and double excited configuration interaction (SDCI) upon an appropriately chosen reference space constructed out of Hartree-Fock-type molecular orbitals. We present SDCI results for energies, wave-function symmetries, bond orders and spin-spin correlations for these states. For the ground state, second order Moller-Plesset (MP2) and size-consistency corrected energies are presented and compared with Monte Carlo results. We also examine the influence of bond length alternation on the calculated properties. It is shown that within the Hubbard model the lowest excited g and u singlet and triplet states of C-60 give very similar overall bond orders and spin-spin correlations. (C) 2000 American Institute of Physics. [S0021-9606(00)30918-7].