Hybrid Quantum Mechanics/Molecular Mechanics: Incorporating Long-Range Electrostatics through Particle-Mesh Ewald Summation

Abstract:

It is well known that solvent interactions can have a profound impact on chemical reactions. In order to capture these interactions in a quantum chemical calculation, hybrid quantum mechanics/molecular mechanics (QM/MM) methods incorporate atomistic point charge representations of the solvent atoms into the QM calculation. Conventional QM/MM methods include solvent atoms within a given radius around the QM system of interest. This truncation can introduce artifacts into the calculation, especially in systems with ions or at interfaces/heterogeneous environments.

We present a method to incorporate electrostatic interactions with the infinite bulk beyond the QM/MM cutoff distance into the QM Hamiltonian. This method (QM/MM/PME) interpolates the potential from the Particle-Mesh Ewald Summation grid employed by standard molecular dynamics packages into the exchange-correlation functional quadrature grid of a DFT calculation. We demonstrate the efficacy of the method in water and in ionic liquid. The method is implemented as an open-source python interface for Psi4 and OpenMM.