The excitation energies of 1,1′-bicyclohexylidene and 1,1′:4′,1″-tercyclohexylidene.: A comparison of multi-reference configuration interaction and perturbation theory approaches Original Research Article

The lowest valence transition energies of the first two homologues of the oligo(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 compared to the previously reported results using a non-selected multi-reference single-double CI (Direct-CI) and a selected multi-reference single-double CI (MRDCI) approach, in order to obtain insight in the applicability of the MRDCI approach for the prediction of absorption spectra of large organic molecules. Calculations of the lowest valence excited states of 1,1′-bicyclohexylidene (1) showed that MR-MP3 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″-tercyclohexylidene (2) revealed that the MR-MP2 methodology is not reliable for the prediction of its transition energies, if the multi-reference character of the excited states increases. Although remarkable differences in the absolute transition energies were found, the assignments of the lowest valence transitions of 1,1′-bicyclohexylidene (1) and 1,1′:4′,1″-tercyclohexylidene (2) were similar at all levels of theory.