An efficient approach to estimate electronic couplings in molecular pairs using molecular orbital grids

The estimation of electronic couplings between diabatic states is crucial for the comprehension of electron transfer phenomena between molecular systems. Therefore, the development of efficient approximations that enable a very fast, yet accurate, estimation of electronic couplings is an important research goal in the context of organic semiconductors. The most popular methods (diabatization schemes, projection approximations, or methods using fragment molecular orbitals) usually involve the use of electronic structure calculations and can be computationally prohibitive if a large number of electronic coupling estimations is required. In this paper, we propose a novel strategy (CubeMap) to evaluate electronic couplings between molecular pairs in an extremely efficient manner. CubeMap employs the well-established linear relationship between the electronic coupling and the overlap integral between the corresponding molecular orbitals localized on the interacting molecules. In particular, CubeMap is based on the efficient calculation of the overlap integral in real space using molecular orbital grids of moderate size. The CubeMap efficiency partly comes from the fact that only a single electronic structure calculation of an individual molecule (reference geometry) is enough for the subsequent evaluation of overlap integrals and electronic couplings in multiple dimer dispositions. We show that CubeMap is particularly appealing to rapidly estimate the electronic coupling distributions in molecular crystals due to thermal fluctuations (dynamic disorder), which is an important effect for the accurate description of charge transport in organic semiconductors. Compared with the methods usually employed to evaluate electronic couplings, the CubeMap approach drastically reduces the computational cost by several orders of magnitude.