ONIOM(QM:QM') Electrostatic Embedding Schemes for Photochemistry in Molecular Crystals

Understanding photoinduced processes in molecular crystals is central to the design of highly emissive materials such as organic lasers and organic light-emitting diodes. The modelling of such processes is, however, hindered by the lack of excited state methodologies tailored for these systems. Embedding approaches based on the Ewald sum can be used in conjunction with excited state electronic structure methods to model the localised excitations which characterise these materials. In this article, we describe the implementation of a two-level ONIOM(QM:QM') point charge embedding approach based on the Ewald method, the Ewald Embedded Cluster (EEC) model. An alternative self-consistent method is also considered to simulate the response of the environment to the excitation. Two molecular crystals with opposing photochemical behaviour were used to benchmark the results with single reference and multireference methods. We observed that the inclusion of an explicit ground state cluster surrounding the QM region was imperative for the exploration of the excited state potential energy surfaces. Using EEC, accurate absorption and emission energies as well as S1-S0 conical intersections were obtained for both crystals. We discuss the implications of the use of these embedding schemes considering the degree of localisation of the excitation. The methods discussed herein are implemented in an open source platform (fromage, https://github.com/Crespo-Otero-group/fromage) which acts as an interface between popular electronic structure codes (Gaussian, Turbomole and Molcas).