A coupled cluster study of the 11A1g and 11B2u states of benzene

A theoretical investigation of the equilibrium structures and harmonic frequencies of the 1 (1)A(1g) and 1 B-1(2u) states of benzene is presented, The performance of coupled cluster singles (CCS), the recently proposed CC2 model, and coupled cluster singles and doubles (CCSD) is compared. The CC2 ground and excited states frequencies are a significant improvement of the CCS results and are relatively close to the CCSD results. A comparative analysis of the vibrations in the two electronic states of both C6H6 and C6D6 is presented. The reliability of predicted shifts in harmonic frequencies between the two states and isotopic shifts is estimated on the basis of the convergence in the CCS, CC2, and CCSD hierarchy of models, and through comparison with related theoretical work. The shifts are used in a critical comparison with experiment. Inconsistent experimental assignments have been scrutinized on the basis of the predicted shifts. The complete harmonic force fields are given for both states. (C) 1998 American Institute of Physics. [S0021-9606(98)02910-9].