Direct Dynamics Reactive Scattering-adaptive QM/MM
The reactive uptake of atmospherically-relevant gases (such as N2O5) by model aerosol surfaces will be simulated using the direct dynamics reactive scattering with adaptive quantum mechanics/molecular mechanics (DDRS-adQM/MM) approach. While the interactions of all molecules in the system would ideally be treated quantum mechanically, a fully-quantum mechanical description is often impractical due to computational expense. Alternatively, the system can be studied at a mixed quantum/classical level, where, for instance, the colliding molecule (N2O5) and the surface water molecules are treated quantum mechanically (QM) and the solvent molecules in the environment region are treated with classical molecular mechanics (MM). Contrary to conventional QM/MM methods, adQM/MM enables a fully QM representation of reactive processes in condensed phases by allowing solvent molecules to diffuse into and out of the active QM region.
Replica Exchange Path-Integral Molecular Dynamics
A key question in atmospheric chemistry concerns the size and composition of the so-called critical cluster, the smallest cluster of molecules that will spontaneously grow by condensation (i.e., without a free-energy barrier). Quantum MD simulations will be performed in parallel with experiments to develop a comprehensive molecular-level understanding of the structural, thermodynamic, and dynamical properties of these nano-particles and to determine the molecular mechanism associated with water uptake. Replica exchange path-integral MD (RE-PIMD) will then be performed to determine, at the quantum-mechanical level, the relative stabilities and populations of the different isomeric structures as a function of temperature and chemical composition.