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

Abstract

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.

Benchmarking Aspects of Ab Initio Fragment Models for Accurate Excimer Potential Energy Surfaces

“…There are various ways to define the point charges; several sets are available, e.g., in traditional molecular force fields like AMBER , or CHARMM . A more flexible parametrization can be achieved using atomic multipoles , as, e.g., within the Effective Fragment Potential (EFP) framework. − In our previous paper on the ground state of noncovalently bound complexes the CHELPG (CHarges from ELectrostatic Potentials using a Grid-based) algorithm was found to work well with CC methods. In the context of CHELPG, the partial charges at the atomic sites are fitted to reproduce the electrostatic potential and some low-order electrostatic moments of the molecule calculated at any level of theory; hence the one used for the active fragment (e.g., CCSD) can also be applied.…”

“… Ĥ 1 e f f is essentially an embedded Hamiltonian which can be used to describe the ground state and also local excited states in the presence of the other fragment. The accuracy of this approach will primarily depend on the choice of V̂ 1 e f f and Δ V 1 ( r 1 , r 2 ); several variants have been tested in our previous study on the ground state . This formalism can also be applied to the excited state of the complex, provided the excitation is localized on one fragment (see e.g.…”

“…DNA, we systematically test the possible components of such fragment methods. As of the interaction of the fragments, in our previous paper several schemes have been tested and we have found that both QM/MM and PbE methods (with a proper account for dispersion and Pauli (exchange) repulsion terms from the extended Effective Fragment Potential (EFP2) − model) are capable of reproducing full dimer calculations of the same level of theory with high fidelity. It is worthwhile to generalize these schemes to excited states.…”

Performance of Multilevel Methods for Excited States

“…Note that eq is only valid when predefined point charges are employed, but the charges can also be generated on-the-fly using the ground-state density matrix of the full system calculation. The point charge representation of the environment usually has moderate effect on the outcome of the calculations, and it is not clear which atomic charge-determination approach performs the best. , As before, the subtraction scheme can be used to obtain the excitation energies of the ONIOM-EE method from the subsystem excitations as where again, the density-matrix dependency of the excitation energies is omitted from the notation. In ONIOM-EE, similar to the ME version, all excitations of the system are described at least at the low level, including the charge transfer ones.…”

“…The point charge representation of the environment usually has moderate effect on the outcome of the calculations, and it is not clear which atomic chargedetermination approach performs the best. 20,51 As before, the subtraction scheme can be used to obtain the excitation energies of the ONIOM-EE method from the subsystem excitations as…”