  New density functional theory approach based on a complete active space
self-consistent field (CASSCF) reference function in Extended Koopmans'
approximation is discussed. Recently, the number of generalizations of density
functional theory based on a multiconfigurational CASSCF reference function
with exact exchange (CASDFT) was introduced. It was shown by one of the authors
(Dr. Gusarov) that such a theory could be formulated by introducing a special
form of exchange-correlation potential. To take into account an active space
and to avoid double counting of correlation energy the dependence from on-top
pair density as a new variable was introduced. Unfortunately, this requires a
deep review and reparametrization of existing functional expressions which lead
to additional computational difficulties. The presented approach does not
require introducing additional variables (like on-top pair density, and based
on Extended Koopmans' theorem (EKT) approximation for multiconfigurational wave
function within CASSCF method.
