The ground state intermolecular potential of bimolecular complexes of N‐heterocycles is analyzed for the impact of individual terms in the interaction energy as provided by various, conceptually different theories. Novel combinations with several formulations of the electrostatic, Pauli repulsion, and dispersion contributions are tested at both short‐ and long‐distance sides of the potential energy surface, for various alignments of the pyrrole dimer as well as the cytosine–uracil complex. The integration of a DFT/CCSD density embedding scheme, with dispersion terms from the effective fragment potential (EFP) method is found to provide good agreement with a reference CCSD(T) potential overall; simultaneously, a quantum mechanics/molecular mechanics approach using CHELPG atomic point charges for the electrostatic interaction, augmented by EFP dispersion and Pauli repulsion, comes also close to the reference result. Both schemes have the advantage of not relying on predefined force fields; rather, the interaction parameters can be determined for the system under study, thus being excellent candidates for ab initio modeling.

The ground state intermolecular potential of bimolecular complexes of N-heterocycles is analysed for the impact of different terms of the interaction energy as provided by various, conceptually different theories. Novel combinations with several formulations of the electrostatic, Pauli repulsion, dispersion and other contributions are tested for a good performance at both short- and long-distance sides of the potential energy surface for various alignments of the pyrrole dimer as well as the cytosine-uracil complex. The integration of a DFT/CC density embedding scheme and dispersion terms from the effective fragment potential (EFP) method is found to provide very good agreement with the reference CCSD(T) potential overall, but a QM/MM approach using CHELPG atomic point charges for the electrostatic interaction augmented by EFP dispersion and Pauli repulsion contributions comes also close. Both of these schemes has the advantage of not relying on predefined force fields, rather the interaction parameters can be obtained for the system under study, therefore excellent candidates for ab initio modeling.

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 approaches are studied on noncovalently bound molecular complexes with interacting chromophores of the fragments, such as π-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 Å.

The performance of multilevel quantum chemical approaches, which utilize an atom-based system partitioning scheme to model various electronic excited states, is studied. The considered techniques include the mechanical-embedding (ME) of “our own N-layered integrated molecular orbital and molecular mechanics” (ONIOM) method, the point charge embedding (PCE), the electronic-embedding (EE) of ONIOM, the frozen density-embedding (FDE), the projector-based embedding (PbE), and our local domain-based correlation method. For the investigated multilevel approaches, the second-order algebraic-diagrammatic construction [ADC(2)] approach was utilized as the high-level method, which was embedded in either Hartree–Fock or a density functional environment. The XH-27 test set of Zech et al. [ 29906111 J. Chem. Theory Comput. 2018 14 4028 ] was used for the assessment, where organic dyes interact with several solvent molecules. With the selection of the chromophores as active subsystems, we conclude that the most reliable approach is local domain-based ADC(2) [L-ADC(2)], and the least robust schemes are ONIOM-ME and ONIOM-EE. The PbE, FDE, and PCE techniques often approach the accuracy of the L-ADC(2) scheme, but their precision is far behind. The results suggest that a more conservative subsystem selection algorithm or the inclusion of subsystem charge-transfers is required for the atom-based cost-efficient methods to produce high-accuracy excitation energies.

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.