A COUPLED-CLUSTER STUDY OF THE 1(1)A(1G) AND 1B-1(2U) 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 pres ented, 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 signific ant improvement of the CCS results and are relatively close to the CCS D results. A comparative analysis of the vibrations in the two electro nic states of both C6H6 and C6D6 is presented. The reliability of pred icted shifts in harmonic frequencies between the two states and isotop ic shifts is estimated on the basis of the convergence in the CCS, CC2 , and CCSD hierarchy of models, and through comparison with related th eoretical work. The shifts are used in a critical comparison with expe riment. 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. [S 0021-9606(98)02910-9].