THE MULTISTATE CASPT2 METHOD

An extension of the multiconfigurational second-order perturbation app roach CASPT2 is suggested, where several electronic states are coupled at second order via an effective-Hamiltonian approach. The method has been implemented into the MOLCAS-4 program system, where it will repl ace the single-state CASPT2 program. The accuracy of the method is ill ustrated through calculations of the ionic-neutral avoided crossing in the potential curves for LiF and of the valence-Rydberg mixing in the V-state of the ethylene molecule. (C) 1998 Elsevier Science B.V. All rights reserved.