  Calculating charge transfer (CT) excitation energies with high accuracy and
low computational cost is a challenging task. Kohn-Sham density functional
theory (KS-DFT), due to its efficiency and accuracy, has achieved great success
in describing ground state problems. To extend to excited state problems, our
group recently demonstrated an approach with good numerical results to
calculate low-lying and Rydberg excitation energies of an $N$-electron system
from a ground state KS or generalized KS calculations of an $(N-1)$-electron
system via its orbital energies. In present work, we explore further the same
methodology to describe CT excitations. Numerical results from this work show
that performance of conventional density functional approximations (DFAs) is
not as good for CT excitations as for other excitations, due to the
delocalization error. Applying localized orbital scaling correction (LOSC) to
conventional DFAs, a recently developed method in our group to effectively
reduce the delocalization error, can improve the results. Overall, the
performance of this methodology is better than time dependant DFT (TDDFT) with
conventional DFAs. In addition, it shows that results from LOSC-DFAs in this
method reach similar accuracy to other methods, such as $\Delta$SCF, $G_0W_0$
with Bethe-Salpeter equations, particle-particle random phase approximation,
and even high-level wavefunction method like CC2. Our analysis show that the
correct $1/R$ trend for CT excitation can be captured from LOSC-DFA
calculations, stressing that the application of DFAs with minimal
delocalization error is essential within this methodology. This work provides
an efficient way to calculate CT excitation energies from ground state DFT.
