Ab initio CASSCF and MRSDCI calculations of the (C6H6)3+ radical

Ab initio complete-active-space self-consistent-field (CASSCF), single-reference singly and doubly excited configuration interaction (SRSDCI), and multi-reference SDCI (MRSDCI) calculations were performed for the benzene trimer cation, (C6H6)(3)(+), in its ground state. We found that the global minimum of the cation is the distorted C-2v sandwich structure, which is 0.032 eV lower than the D-6h sandwich structure. The dissociation energy (D-e) relative to (C6H6)(2) + C6H6 was calculated to be 0.43 eV, in comparison to the experimental value (D-0) of 0.34 +/-0.02 eV. Our calculations revealed that most of the charge of the trimer cation is localized in the central benzene ring, whose gross charge is + 0.9. The low-lying excited states arising from the pi- pi transition are also discussed. (C) 2000 Elsevier Science B.V. All rights reserved.