Mono and multiconfigurational wave functions with DFT correlation energy: the case of fluorine

It is known that the Hartree-Fock-Kohn-Sham method does not constitute an i mprovement over the Hartree-Fock method when the latter gives a poor descri ption of the system, as is the case for the fluorine molecule. On the contr ary, it is known that a multiconfigurational (MC) wave function gives a rat her good description of this molecule. We show that adding the correlation energy computed from a variety of correlation energy functionals to the MC energy improves the potential energy curve and the spectroscopic constants significantly. We consider two different ways of including the correlation energy, either in a self-consistent (SCF) fashion or through a post-SCF pro cedure. (C) 2000 Elsevier Science B.V. All rights reserved.