Role of the configuration interaction in the CNDO/S calculation of optical rotatory strengths

The rotatory strengths calculated directly by the CNDO/S method exhibit a pronounced dependence on the size of configuration interaction (CI). In order to elucidate the role of highly excited configurations in such calculations the perturbation theory is employed. It is shown that the restriction of the CI size to 20–40 may be quite inadequate in some cases. The calculations of rotatory strengths of several optically active molecules containing carbonyl and amide chromophores has shown that the best results can be obtained for half of full CI but sometimes it is possible to restrict the CI size to 100 configurations. The agreement with experiment for all molecules considered is satisfactory.