Benchmarking aspects of ab initio fragment models for accurate excimer potential energy surfaces

10 May 2023, Version 4
This content is a preprint and has not undergone peer review at the time of posting.


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 the degree for which these methods can be applied. In this work different aspects of fragment-based approach are studied on non-covalently bound molecular complexes with interacting chromophores of the fragments, such as pi-stacked nucleobases. The interaction of the fragments is considered at two distinct steps. First, the states localized on the fragments are described in the presence of the other fragment(s); for this we test two approaches. One method is founded on QM/MM principles, only including the electrostatic interaction between the fragments in the electronic structure calculation with Pauli repulsion and dispersion effects 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 adequate 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 provides accurate splitting of the energies of interacting chromophores that are separated by more than 4 Angstrom.


excited states
exciton coupling
intermolecular interactions
Effective Fragment Potential
Pauli repulsion

Supplementary materials

Supporting Information
Structures of the monomers and complexes evaluated in this study, ground state potential curves, the impact of “exact” ground state van-der-Waals terms on the homodimer excited state surfaces, comparison of the CCSD and CC2 excited state splittings in the (CH2O)2 system, excited state total energy curves of (CH2O)2 as well as the reference CCSD data for all studied states are presented here. In addition, the failure of the localization of virtual orbitals are demonstrated by showing a Rydberg orbital obtained in the PbE procedure at different intermolecular distances. Details of the calculation of the CHelpG charges and their values used in this study are also given.


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