Definition and benchmarking of ab initio fragment methods for accurate excimer potential energy surfaces

While Coupled-Cluster methods have been proven to provide an accurate description of excited electronic states, the scaling of the computational costs with the system size limits their applicability. In this study a fragment-based approach is presented which can be applied to non-covalently bound molecules with interacting chromophores localized on the fragments (so called Frankel pairs), such as $\pi$-stacked nucleobases. The interaction of the fragments is considered at two distinct points. First, the states localized on the fragments are described in the presence of the other fragment(s), for which we test two approaches. A QM/MM type method that only includes the electrostatic interaction between the fragments in the electronic structure calculations, while Pauli repulsion and dispersion effects are added separately. The other model, a Projection-based Embedding (PbE) using the Huzinaga equation includes both electrostatic and Pauli repulsion and only needs to be augmented by dispersion interactions. In both schemes the extended Effective Fragment Potential (EFP2) method of Gordon et al.~was found to provide an accurate correction for the missing terms. In the second step, the interaction of the localized chromophores is modeled for a proper description of the excitonic coupling. Here the inclusion of purely electrostatic contributions appears to be sufficient: it is found that the Coulomb part of the coupling, as evaluated using the Transition Density Cube method, provides accurate splitting of the energy of interacting chromophores above 4 \AA\ intermolecular separations.