  The alternative separation of exchange and correlation energies proposed by
Toulouse et al. [Theor. Chem. Acc. 114, 305 (2005)] is explored in the context
of multi-configuration range-separated density-functional theory. The new
decomposition of the short-range exchange-correlation energy relies on the
auxiliary long-range interacting wavefunction rather than the Kohn-Sham (KS)
determinant. The advantage, relative to the traditional KS decomposition, is
that the wavefunction part of the energy is now computed with the regular
(fully-interacting) Hamiltonian. One potential drawback is that, because of
double counting, the wavefunction used to compute the energy cannot be obtained
by minimizing the energy expression with respect to the wavefunction
parameters. The problem is overcome by using short-range optimized effective
potentials (OEPs). The resulting combination of OEP techniques with
wavefunction theory has been investigated in this work, at the Hartree-Fock
(HF) and multi-configuration self-consistent-field (MCSCF) levels. In the HF
case, an analytical expression for the energy gradient has been derived and
implemented. Calculations have been performed within the short-range local
density approximation on H2, N2, Li2 and H2O. Significant improvements in
binding energies are obtained with the new decomposition of the short-range
energy. The importance of optimizing the short-range OEP at the MCSCF level
when static correlation becomes significant has also been demonstrated for H2,
using a finite-difference gradient. The implementation of the analytical
gradient for MCSCF wavefunctions is currently in progress.
