M. Reguero et al., CALCULATION OF SMALL TRANSITION ENERGIES FROM A DRESSED CONFIGURATION-INTERACTION, Chemical physics letters, 265(6), 1997, pp. 621-628
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 excit
ations. 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 impo
rtance of removing unlinked terms from these CI spaces is emphasised.