Dynamic QM/MM: A hybrid approach to simulating gas-liquid interactions

In this chapter we describe molecular dynamics simulation methods in which the system being studied is divided into a region where quantum mechanics (QM) is used to determine forces for doing Born-Oppenheimer direct dynamics calculations (i.e., doing electronic structure calculations on the fly to determine energies and forces) and another region where empirical potentials that are commonly used in molecular mechanics (MM) calculations are used to determine forces. The two regions are linked through an embedding process that may or may not involve the possibility that atoms can be passed back and forth between regions at each time step. The idea with this dynamic QM/MM methodology is that one uses QM calculations to define the potential surface in portions of the system where reaction occurs, and MM to determine forces in what is typically a much larger region where no reaction occurs. This approach thereby enables the description of chemical reactions in the QM region, which is a technology that can be used in many different applications. We illustrate its use by describing work that we have done with gas-liquid reactions in which a reactive atom (such as an oxygen or fluorine atom) reacts with the surface of a liquid and the products can either remain in the liquid or emerge into the gas phase. Applications to hydrocarbon and ionic liquids are described, including the characterization of reaction mechanisms at hyperthermal energies, and the determination of product branching and product energy distributions.