Computational Chemistry

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Computational Chemistry

Park, K., Go, A. W., Walker, R. C., & Paesani, F.,
J. Chem. Theory Comput., 2012, 8(2868).

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.

Vibrational Spectroscopy

Paesani, J. Phys. Chem. A, 2011, 115(25)

Molecular-level insights on the structural and dynamical properties of the interfaces will be gained from integration of spectroscopic measurements and molecular simulations. Recently, the Paesani group has developed a fully “first principles” simulation approach to model the structure, thermodynamic, dynamical, and spectroscopic properties of water under different conditions. Excellent agreement between the simulation results and the available experimental data obtained over a range of temperatures from the melting point up to near the evaporation point. In particular, the theoretically predicted energy barriers and time scales associated with the hydrogen-bond dynamics are found to be comparable to the experimental values obtained from two-dimensional and pump−probe infrared spectra.

Paesani, F.; Temperature-dependent infrared spectroscopy from a first principles approach, J. Phys. Chem. A, 2011, 115(25).

Johnston, J. C., & Molinero, V.,
J. Am. Chem. Soc., 2012, 134(6650)

Coarse Graining

On a molecular level, the nucleation of ice is a rare event, making its study through atomistic simulations extremely challenging. The Molinero Group has developed a computationally efficient molecular model of water (termed mW) that enables accurate simulation of ice nucleation. The coarse-grained mW model demonstrates, for example, that structural transformations of supercooled liquid water into a mostly four-coordinated liquid control the rate of homogeneous ice nucleation. This provides a microscopic foundation for the correlation between water activity and freezing rates. Recently, the mW model has been used to study homogeneous ice nucleation in aqueous salt solutions. In Phase II, these methodologies will be extended to the study of nucleation in increasingly complex systems, such as organic/water and salt/organic/water solutions and mineral surfaces.