(168a) Free-Energy Perturbation: Application to Chemical Reactions At Metal/Water Interfaces

Faheem, M., University of South Carolina
Heyden, A., University of South Carolina

Computational investigations of structural and energetic changes in chemical reactions at metal/water interfaces pose a unique challenge of properly accounting for the effect of liquid-phase environment on free energies of reactants, products and transition states while keeping the computational model simple enough to be computationally affordable. A practical solution to this challenge is a hybrid QM/MM approach. By treating only the reaction zone and its immediate environment quantum mechanically (QM) for higher accuracy and the remainder of the metal surface and the bulk of the liquid water at the classical, molecular mechanical (MM) level of theory, a reasonably accurate energetic description of the complex metal/water system can be obtained while simultaneously realizing a computational speedup of multiple orders of magnitude.

In this talk, we present a novel solvation scheme that combines planewave density functional theory (DFT), periodic electrostatic embedded cluster (PEEC) calculations with Gaussian-type orbitals, and classical molecular dynamics (MD) calculations with the QM/MM free-energy perturbation (FEP) methodology to enable an accurate determination of free-energies of reaction and activation free-energy barriers at periodic metal/water interfaces. Optimization of local conformations (i.e., reactants, products, and transition states) are performed using an iterative approach with sequential MD sampling and QM optimization steps to reduce the number of QM calculations and the amount of MD sampling. Furthermore, we use a fixed-size, finite ensemble of MM conformations to allow a precise evaluation of the QM potential of mean force (PMF) and its gradient defined within this ensemble. As a result, we circumvent the challenges associated with statistical averaging during MD sampling and speed up the convergence of the optimization process. Application to C-C cleavage in double-dehydrogenated ethylene glycol on Pt (111) surface will be discussed as model reaction.