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Mutually polarizable QM/MM model with in situ optimized localized basis functions

Mutually polarizable QM/MM model with in situ optimized localized basis functions
Mutually polarizable QM/MM model with in situ optimized localized basis functions
We extend our recently developed quantum-mechanical/molecular mechanics (QM/MM) approach [Dziedzic et al., J. Chem. Phys. 145, 124106 (2016)] to enable in situ optimization of the localized orbitals. The quantum subsystem is described with onetep linear-scaling density functional theory and the classical subsystem – with the AMOEBA polarizable force field. The two subsystems interact via multipolar electrostatics and are fully mutually polarizable. A total energy minimization scheme is employed for the Hamiltonian of the coupled QM/MM system. We demonstrate that, compared to simpler models using fixed basis sets, the additional flexibility offered by in situ optimized basis functions improves the accuracy of the QM/MM interface, but also poses new challenges, making the QM subsystem more prone to overpolarization and unphysical charge transfer due to increased charge penetration. We show how these issues can be efficiently solved by replacing the classical repulsive van der Waals term for QM/MM interactions with an interaction of the electronic density with a fixed, repulsive MM potential that mimics Pauli repulsion, together with a modest increase in the damping of QM/MM polarization. We validate our method, with particular attention paid to the hydrogen bond, in tests on water-ion pairs, the water dimer, first solvation shells of neutral and charged species, and solute-solvent interaction energies. As a proof of principle, we determine suitable repulsive potential parameters for water, K+, and Cl−. The mechanisms we employed to counteract the unphysical overpolarization of the QM subsystem are demonstrated to be adequate, and our approach is robust. We find that the inclusion of explicit polarization in the MM part of QM/MM improves agreement with fully QM calculations. Our model permits the use of minimal size QM regions and, remarkably, yields good energetics across the well-balanced QM/MM interface.
0021-9606
Dziedzic, Jacek
8e2fdb55-dade-4ae4-bf1f-a148a89e4383
Head-Gordon, Teresa
11febdf4-20fa-4abb-97a1-3305c6e81b09
Head-Gordon, Martin
f203c934-60ff-4c19-a2ff-b1f6ec2ad05a
Skylaris, Chris-Kriton
8f593d13-3ace-4558-ba08-04e48211af61
Dziedzic, Jacek
8e2fdb55-dade-4ae4-bf1f-a148a89e4383
Head-Gordon, Teresa
11febdf4-20fa-4abb-97a1-3305c6e81b09
Head-Gordon, Martin
f203c934-60ff-4c19-a2ff-b1f6ec2ad05a
Skylaris, Chris-Kriton
8f593d13-3ace-4558-ba08-04e48211af61

Dziedzic, Jacek, Head-Gordon, Teresa, Head-Gordon, Martin and Skylaris, Chris-Kriton (2019) Mutually polarizable QM/MM model with in situ optimized localized basis functions. The Journal of Chemical Physics, 150 (7). (doi:10.1063/1.5080384).

Record type: Article

Abstract

We extend our recently developed quantum-mechanical/molecular mechanics (QM/MM) approach [Dziedzic et al., J. Chem. Phys. 145, 124106 (2016)] to enable in situ optimization of the localized orbitals. The quantum subsystem is described with onetep linear-scaling density functional theory and the classical subsystem – with the AMOEBA polarizable force field. The two subsystems interact via multipolar electrostatics and are fully mutually polarizable. A total energy minimization scheme is employed for the Hamiltonian of the coupled QM/MM system. We demonstrate that, compared to simpler models using fixed basis sets, the additional flexibility offered by in situ optimized basis functions improves the accuracy of the QM/MM interface, but also poses new challenges, making the QM subsystem more prone to overpolarization and unphysical charge transfer due to increased charge penetration. We show how these issues can be efficiently solved by replacing the classical repulsive van der Waals term for QM/MM interactions with an interaction of the electronic density with a fixed, repulsive MM potential that mimics Pauli repulsion, together with a modest increase in the damping of QM/MM polarization. We validate our method, with particular attention paid to the hydrogen bond, in tests on water-ion pairs, the water dimer, first solvation shells of neutral and charged species, and solute-solvent interaction energies. As a proof of principle, we determine suitable repulsive potential parameters for water, K+, and Cl−. The mechanisms we employed to counteract the unphysical overpolarization of the QM subsystem are demonstrated to be adequate, and our approach is robust. We find that the inclusion of explicit polarization in the MM part of QM/MM improves agreement with fully QM calculations. Our model permits the use of minimal size QM regions and, remarkably, yields good energetics across the well-balanced QM/MM interface.

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Accepted/In Press date: 28 January 2019
e-pub ahead of print date: 19 February 2019

Identifiers

Local EPrints ID: 428732
URI: https://eprints.soton.ac.uk/id/eprint/428732
ISSN: 0021-9606
PURE UUID: c202bbeb-decd-4be3-a02a-3d2a9e251914
ORCID for Jacek Dziedzic: ORCID iD orcid.org/0000-0003-4786-372X
ORCID for Chris-Kriton Skylaris: ORCID iD orcid.org/0000-0003-0258-3433

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Date deposited: 07 Mar 2019 17:30
Last modified: 15 Aug 2019 00:43

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Contributors

Author: Jacek Dziedzic ORCID iD
Author: Teresa Head-Gordon
Author: Martin Head-Gordon

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