The excitation energies of 1,1 '-bicyclohexylidene and 1,1 ': 4 ', 1 ''-tercyclohexylidene. A comparison of multi-reference configuration interactionand perturbation theory approaches

The lowest valence transition energies of the first two homologues of the o ligo(cyclohexylidene) series, viz. 1,1'-bicyclohexylidene (1) and 1,1':4',1 "-tercyclohexylidene (2), were computed using multi-reference perturbation theory at the second (MR-MP2) and third order (MR-MP3). The results were co mpared to the previously reported results using a non-selected multi-refere nce single-double CI (Direct-CI) and a selected multi-reference single-doub le Ct (MRDCI) approach, in order to obtain insight in the applicability of the MRDCI approach for the prediction of absorption spectra of large organi c molecules. Calculations of the lowest valence excited states of 1,1'-bicy clohexylidene (1) showed that MR-MPS gives energies comparable to those of Direct-CI; for the transition energies, MR-MP2 and MR-MP3 performed equally well. The calculations on the excited states of 1,1':4',1"-tercyclohexylid ene (2) revealed that the MR-MP2 methodology is not reliable for the predic tion of its transition energies, if the multi-reference character of the ex cited states increases. Although remarkable differences in the absolute tra nsition energies were found, the assignments of the lowest valence transiti ons of 1,1'-bicyclohexylidene (1) and 1,1':4',1"-tercyclohexylidene (2) wer e similar at all levels of theory. (C) 1999 Elsevier Science B.V. All right s reserved.