Supercell convergence of charge-transfer energies in pentacene molecular crystals from constrained DFT

TL;DR

This paper introduces an electrostatically-corrected constrained DFT method to accurately and efficiently compute charge-transfer energies in pentacene molecular crystals, accounting for screening effects and enabling large system studies.

Contribution

A novel electrostatic correction for constrained DFT is developed, improving charge-transfer energy calculations in molecular crystals like pentacene.

Findings

Method accurately estimates CT energies in infinite crystals.

Validation shows cluster approximation effectiveness.

Approach scalable to large systems.

Abstract

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.