Description of quasidegenerate electronic states exhibiting avoided crossing

The feasibility of the multiconfiguration quasidegenerate perturbation theory (MCQDPT) with improved virtual orbital-complete active space configuration interaction (IVO-CASCI) reference function is demonstrated by scrutinising the interaction potential of LiCl and Al3+ . . . X (X=He and Ne) and analysing the avoided crossing along the bond dissociation, representing a hard challenge for any given electronic structure approach. IVO-MCQDPT procedure is capable of providing very close agreement of the computed energy surfaces and spectroscopic parameters with the reference values at the expense of low computational labour showing that the present method is valuable for dealing with the avoided curve crossing problem. [GRAPHICS] .