Comparison of approximate intermolecular potentials for ab initio fragment calculations on medium sized N-heterocycles

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.