Prediction of the aqueous redox properties of functionalized quinones using a new QM/MM variational formulation

TL;DR

This paper introduces a new variational formulation for modeling solvent effects on quantum systems, specifically applied to predict the redox properties of functionalized quinones using a coupled QM/MM approach.

Contribution

It presents a novel variational framework for mixed quantum-classical systems that simplifies calculations by depending only on the nuclear density matrix.

Findings

The method accurately predicts aqueous redox properties of quinones.

The approach effectively incorporates solvent effects into quantum calculations.

It enables efficient geometry optimization in mixed systems.

Abstract

We recently proposed a method coupling quantum mechanics (QM) methods and molecular density functional theory (MDFT) to describe mixed quantum-classical systems [J. Chem. Phys. 161, 014113 (2024)]. This approach is particularly appropriate to account for solvent effect into QM calculations. We introduce a new variational formulation for the grand potential of a mixed quantum-classical system. Within the Born-Oppenheimer approximation and neglecting electronic entropy, the quantum solute is described by a product of electronic and nuclear density matrices, both depending parametrically on coordinates of the classical solvent. It can then be shown that a functional of the total density matrix satisfies a variational principle for the grand potential. Using a mean-field approximation, we express the grand potential of the mixed quantum-classical system as a variational problem which depends only on the nuclear density matrix, which experiences an external field generated by the electronic and classical one-particle densities. In practice, the grand potential is computed by a series of coupled classical and quantum DFT calculations, together with geometry optimization.