  Photoinduced charge-transfer processes play a key role for novel photovoltaic
phenomena and devices. Thus, the development of ab initio methods that allow
for an accurate and computationally inexpensive treatment of charge-transfer
excitations is a topic that nowadays attracts a lot of scientific attention. In
this paper we extend an approach recently introduced for the description of
single and double excitations (M. Tassi, I. Theophilou and S. Thanos, Int. J.
Quantum Chem., (113), 690 (2013), M. Tassi, I. Theophilou and S. Thanos, J.
Chem. Phys. (138), 124107 (2013)) to allow for the description of
intermolecular charge-transfer excitations. We describe an excitation where an
electron is transferred from a donor system to an acceptor one, keeping the
excited state orthogonal to the ground state and avoiding variational collapse.
These conditions are achieved by decomposing the space spanned by the
Hartree-Fock (HF) ground state orbitals into four subspaces: The subspace
spanned by the occupied orbitals that are localized in the region of the donor
molecule, the corresponding for the acceptor ones and two more subspaces
containing the virtual orbitals that are localized in the neighborhood of the
donor and the acceptor, respectively. Next, we create a Slater determinant with
a hole in the subspace of occupied orbitals of the donor and a particle in the
virtual subspace of the acceptor. Subsequently we optimize both the hole and
the particle by minimizing the HF energy functional in the corresponding
subspaces. Finally, we test our approach by calculating the lowest
charge-transfer excitation energies for a set of tetracyanoethylene-hydrocarbon
complexes, that have been used earlier as a test set for such kind of
excitations.
