A coupled density functional-molecular mechanics Monte Carlo simulation method: The water molecule in liquid water

Abstract

A theoretical model to investigate chemical processes in solution is described. It is based on the use of a coupled density functional/molecular mechanics Hamiltonian. The most interesting feature of the method is that it allows a detailed study of the solute's electronic distribution and of its fluctuations. We present the results for isothermal‐isobaric constant‐NPT Monte Carlo simulation of a water molecule in liquid water. The quantum subsystem is described using a double‐zeta quality basis set with polarization orbitals and nonlocal exchange‐correlation corrections. The classical system is constituted by 128 classical TIP3P or Simple Point Charge (SPC) water molecules. The atom‐atom radial distribution functions present a good agreement with the experimental curves. Differences with respect to the classical simulation are discussed. The instantaneous and the averaged polarization of the quantum molecule are also analyzed. © 1996 by John Wiley & Sons, Inc.

Structure and Ultrafast X-ray Diffraction of the Hydrated Metaphosphate

“…Generally, the fluctuation of reactive subsystems calculated by time-consuming QM methods is weaker than that of the surrounding system described by the MM approach due to the relatively smaller size of the embedded QM subsystem. Taking advantage of its efficiency and capability of capturing the quantum mechanical nature of valence bonds, the QM/MM approach has been widely used in the study of molecules in solution, e.g., simulation of solvatochromic shifts − and finding minimal free energy reaction pathways. − Thus, instead of constructing the thermal ensemble by carrying out QM/MM calculations for every phase-space conformation generated by the Monte Carlo simulation, we only perform QM/MM optimizations on much fewer uncorrelated samples to reach convergence of the Monte Carlo simulation. Our HS-QM/MM-MC method performs the Monte Carlo sampling of thermal ensembles with every uncorrelated phase-space conformation optimized at the QM/MM level and further enables the interrogation of reaction mechanisms in solution at a finite temperature.…”