Combining Multilevel Hartree Fock and Multilevel Coupled Cluster with Molecular Mechanics: a Study of Electronic Excitations in Solutions

TL;DR

This paper explores a hybrid quantum mechanics/molecular mechanics approach combining multilevel Hartree Fock and coupled cluster methods with molecular mechanics to accurately predict electronic excitations in solvated molecules, balancing computational cost and accuracy.

Contribution

It introduces a three-layer coupling scheme integrating multilevel quantum methods with molecular mechanics, improving the modeling of solute-solvent interactions in electronic excitation calculations.

Findings

Achieved near-perfect agreement with experimental excitation energies.

Demonstrated the importance of including specific solute-solvent interactions.

Reduced computational cost by treating long-range interactions classically.

Abstract

We investigate the coupling of different quantum-embedding approaches with a third molecular-mechanics layer, which can be either polarizable or non-polarizable. In particular, such a coupling is discussed for the multilevel families of methods, in which the system is divided into an active and an inactive orbital spaces. The computational cost of the resulting three layers approaches is reduced by treating the long-range interactions at the classical level.The methods developed are tested against the calculation of the excitation energies of molecular systems in aqueous solution, for which an atomistic description of the environment is crucial to correctly reproduce the specific solute-solvent interactions, such as hydrogen bonding. In particular, we present the results obtained for three different moieties, acrolein, pyridine and para-nitroaniline, showing that an almost perfect agreement with experimental data can be achieved when the relevant physico-chemical interactions are included in the modeling of the condensed phase.