A Simple and Effective Solution to the Constrained QM/MM Simulations

TL;DR

This paper introduces a simple boundary constraint with correction (BCC) method to prevent solvent diffusion in QM/MM simulations, improving accuracy while maintaining physical properties, demonstrated on aqueous hydronium ion.

Contribution

The paper presents a novel BCC method that effectively prevents solvent diffusion in QM/MM simulations with minimal bias, ensuring physical consistency and accurate results.

Findings

The BCC method successfully prevents solvent diffusion in QM/MM simulations.

The correction term accurately compensates bias, aligning RDF with ab initio results.

The method maintains energy and force continuity, conserving physical properties.

Abstract

It is a promising extension of the quantum mechanical/molecular mechanical (QM/MM) approach to incorporate the solvent molecules surrounding the QM solute into the QM region to ensure the adequate description of the electronic polarization of the solute. However, the solvent molecules in the QM region inevitably diffuse into the MM bulk during the QM/MM simulation. In this article we developed a simple and efficient method, referred to as boundary constraint with correction (BCC), to prevent the diffusion of the solvent water molecules by means of a constraint po- tential. The point of the BCC method is to compensate the error in a statistical property due to the bias potential by adding a correction term obtained through a set of QM/MM simulations. The BCC method is designed so that the effect of the bias potential completely vanishes when the QM solvent is identical with the MM solvent. Furthermore, the desirable conditions, that is, the continuities of energy and force and the conservations of energy and momentum, are fulfilled in principle. We applied the QM/MM-BCC method to a hydronium ion in aqueous solution to construct the radial distribution function(RDF) of the solvent around the solute. It was demonstrated that the correction term fairly compensated the error and led the RDF in good agreement with the result given by an ab initio molecular dynamics simulation.