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 C2v sandwich structure, which is 0.032 eV lower than the D6h sandwich structure. The dissociation energy (De) relative to (C6H6)2++C6H6 was calculated to be 0.43 eV, in comparison to the experimental value (D0) 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 π–π transition are also discussed.