Polarizable Embedding QM/MM for Periodic Systems

TL;DR

This paper introduces a polarizable embedding QM/MM scheme for periodic systems that accurately models mutual polarization, enabling efficient and smooth convergence in simulations of water molecules.

Contribution

It presents a novel PE-QM/MM method coupling DFT with a multipole expansion model for periodic systems, improving accuracy and convergence.

Findings

Smooth convergence of the periodic interaction potential.

Accurate matching of PE-QM/MM with pure QM calculations.

Effective damping prevents over-polarization in simulations.

Abstract

A general polarizable embedded (PE) quantum mechanics/molecular mechanics scheme for periodic systems is presented, describing mutual polarization of the two subsystems. The QM system, described with density functional theory (DFT), is coupled to a single center multipole expansion (SCME) model, characterising H$_2$O molecules in the MM region. In SCME the H$_2$O molecules are ascribed anisotropic dipole and quadrupole polarizabilities and permanent multipoles up to and including the hexadecapole. Our embedding scheme illustrates a smooth and efficient convergence pattern of the periodic interaction potential by introducing a single and clustered multipole expansion points in the far-field. By choosing the near- and far-field expansion of the potential carefully the PE-QM/MM calculation matches the level of accuracy of a the QM calculation. In the short range, the electrostatic interaction between the QM and MM subsystems is damped with a real-space and pair-wise isotropic damping functions - resulting in a screened interaction and preventing over-polarization. In molecular dynamics simulations the two subsystems are separated with the elastic scattering assisted flexible inner region [Kirchhoff et. al. JCTC, 2021, 17, 9, 5863] - ensuring a smooth transition in the radial distribution at the boundary between the two subsystems.