Computational investigations of structural and energetic changes in chemical reactions at a metal-water interface pose a unique challenge of properly accounting for the effect of an aqueous environment on free energies of reactants, products and transition states while maintaining computational affordability of the model. A practical solution to this challenge is a hybrid quantum mechanics/molecular mechanics (QM/MM) approach that can provide a reasonably accurate energetic description of the complex metal-water system while offering a computational speedup of multiple orders of magnitude.

            In this talk, we discuss the development of a QM/MM free energy perturbation (FEP) method for modeling heterogeneously catalyzed chemical reactions at metal-water interfaces. This novel QM/MM framework combines planewave density function theory (DFT), periodic electrostatic embedded cluster method (PEECM) calculations using Gaussian-type orbitals, and classical molecular dynamics (MD) simulations to obtain an accurate energetic description of the complex metal-water system. We derive a potential of mean force (PMF) description of the reaction system within the QM/MM framework. A fixed-size, finite ensemble of MM conformations is used to permit a precise evaluation of the PMF of QM coordinates and its gradient defined within this ensemble. Local conformations of adsorbed reaction moieties are optimized using sequential MD-sampling and QM-optimization steps. The reaction coordinate is constructed using sufficient number of interpolated states, and free energy difference between adjacent states is calculated using QM/MM-FEP method. By avoiding on-the-fly QM calculations and by circumventing the challenges associated with statistical averaging during MD sampling, a computational speedup of multiple orders of magnitude is realized. The method is systematically validated against the results of ab initio QM calculations and demonstrated for C-C cleavage in double-dehydrogenated ethylene glycol on Pt (111) surface.