  Singlet fission (SF) is a multi-exciton generation process that could be
harnessed to improve the efficiency of photovoltaic devices. Experimentally,
systems derived from the pentacene molecule have been shown to exhibit
ultrafast SF with high yields. Charge-transfer (CT) configurations are likely
to play an important role as intermediates in the SF process in these systems.
In molecular crystals, electrostatic screening effects and band formation can
be significant in lowering the energy of CT states, enhancing their potential
to effectively participate in SF. In order to simulate these, it desirable to
adopt a computational approach which is acceptably accurate, relatively
inexpensive, which and scales well to larger systems, thus enabling the study
of screening effects. We propose a novel, electrostatically-corrected
constrained Density Functional Theory (cDFT) approach as a low-cost solution to
the calculation of CT energies in molecular crystals such as pentacene. Here we
consider an implementation in the context of the ONETEP linear-scaling DFT
code, but our electrostatic correction method is in principle applicable in
combination with any constrained DFT implementation, also outside the
linear-scaling framework. Our newly developed method allows us to estimate CT
energies in the infinite crystal limit, and with these to validate the accuracy
of the cluster approximation.
