Many photochemical processes, such as catalytic processes or the generation of charges in active layer heterostructures of organic solar cells, involve the transfer of an electron triggered by the absorption of a photon. Such photoinduced electron transfer reactions are typically influenced by a variety of properties, ranging from the intrinsic molecular electronic structure of the molecular building blocks of the material, the details of the local mutual arrangement of molecules, to larger scale morphological ordering. In many situations, the inherent disorder of the material systems in which the electron transfer takes place suggests the use of localized diabatic states to describe the reactions and to map the effects of the local and global environment onto them. This idea as given rise to multiscale simulation approaches, in which the transport of excitations across a material is modeled as a series of bi-molecular transfer events, each of them described by an effective transfer rate. According to Marcus theory, in the non-adiabatic high temperature or activated crossing limit, the rate of electronic excitation transfer between two states X (initial) and Y (final) is