QM/MM dynamics with an explicit, flexible solvent shell

J. VandeVondele, M.C. Colombo, L. Guidoni, A. Laio, and U. Rothlisberger

University of Cambridge,

Ecole polytechnique de Laussane

Swiss center for scientific computing

QM/MM simulations are a powerful tool for studying reactive events in complex, condensed phase systems. This approach requires that a small, reactive subsystem be embedded in a large environment. An accurate QM method can be used to describe the reactive core and nearby atoms, while a more expedient classical method is used for the bulk. A significant advantage of this method is that accuracy can easily be verified and improved by increasing the size of the QM subsystem.

Solute / solvent interactions are important and of significant chemical interest. Pair correlation functions and other solvation patterns, solvent shifts in electronic spectra, and solvent-solute chemical reactions can all be studied with QM/MM methods. These properties require averages over long trajectories of QM/MM dynamics. Additionally, for many of these properties, it is highly advantageous or even required that part of the solvent, e.g. the first solvation shell, be treated as a QM system. However, this poses a problem for many QM/MM dynamics simulations, as the molecules that solvate the solute will change over time (molecules will diffuse away from the solute, while other molecules approach), and the number of molecules that actually is in contact with the solute is likely to fluctuate.

A scheme will be presented that adresses this issue and allows for dynamics with an explicit and flexible solvent shell. Within this scheme, the number of molecules in close contact with a solute is allowed to change. At the same time, QM molecules can not diffuse far from the solute and MM molecules can not come too close so that high accuracy and and a compact QM subsystem are guaranteed throughout. We will show that this scheme guarantees correct thermodynamic sampling, provided that the QM/MM potential is accurate.