Calculation of small transition energies from a dressed configuration interaction

Small energy separations between nearly degenerate states in magnetic systems or mixed valence complexes are difficult to calculate. In the recent past difference dedicated CI (DDCI) calculations appeared to be efficient but they omit the 3rd-order effect of inactive double excitations. A size-consistent dressed CI technique, applied on CAS single and double CI spaces, on model problems, is shown to treat efficiently these effects and to give better results than DDCI. The relative importance of removing unlinked terms from these CI spaces is emphasised.