QM/MM studies of gas-liquid collisional energy transfer

In this manuscript we describe a molecular dynamics method that has recently been developed to model collisional energy transfer in collisions of hyperthermal gas-phase molecules with ionic liquid surfaces. This method is based on a quantum mechanics/molecular mechanics (QM/MM) partitioning in which electronic structure methods are used to determine forces for the hyperthermal gaseous molecule and molecules in the liquid which it directly interacts with during the first 10 ps of the collision process, while empirical potentials are used for atoms in the liquid that are not directly involved in interacting with the gaseous molecule. Collisions of the gaseous molecule with the liquid surface are simulated using molecular dynamics methods, and from this it is possible to determine energy transfer associated with the gas-surface collision, including scattering of the molecule from the liquid that leads to energy and momentum transfer, and trapping of the gas molecule in the liquid. We illustrate this approach with an application to the collision of hyperthermal CO2 with two ionic liquid surfaces: [bmim][BF4] and [bmim][Tf2N]. The results are used to model recent experimental studies, and our analysis includes an examination of rotational and angular distributions associated with the CO2 scattering, and the translational diffusion and vibrational relaxation of the trapped CO2.