The role of the doubly excited configurations in the ?-electronic structure of the croconate anion

Abstract

The electronic structure of the croconate anion is investigated by the semi-empirical VESCF method with CI including up to the doubly excited configurations. The effect of the doubly excited configuration is quite large in the lowest singlet state which turns out to be a ground state, while the calculation including only singly excited configurations produces a triplet ground state. The calculated first transition energy (1A′1 →1E′1) is in fair agreement with the observed value. The interpretation of the transition moment in case that the doubly excited configurations are included is also discussed.