Solvation free energies for periodic surfaces: comparison of implicit and explicit solvation models

Abstract

The evaluation of solvation energies is a great challenge. We focus here on an organic molecule chemisorbed at a metal-liquid interface, as a prototypical system, essential in tribology, electrochemistry and heterogeneous catalysis. We compare an established implicit solvation scheme with a strategy based on molecular mechanics (MM) free energy perturbation (FEP) seeded by QM computations. First, we benchmark the approaches against experimental hydration energies of standard (organic) molecules and find acceptable errors in the order of 0.06 eV (1.3 kcal/mol). Then, the impact of various parameters on the solvation energy of an adsorbate have been assessed on a typical system of interest, levulinic acid adsorbed at a Ru(0001)/water interface. We identify the need for dipole corrections or symmetric slabs when including solvation effects on metallic surfaces. The MM-FEP scheme is revealed to be as reliable as the implicit solvent for water. In the case of levulinic acid, both PCM and MM-FEP agree that the bulk solvation effect is not sufficient to change the adsorption mode from bidentate to mono-dentate, despite the fact that the COOH group is desolvated in the bidentate case. MM-FEP has the great advantage of being more easily generalized to other solvents and to be further improved which will be particularly useful to study solvent and (counter-)ion effects on interfacial reactions.

Practical Considerations for Continuum Models Applied to Surface Electrochemistry

“… is printed in VASPsol as the negative of the output flag FERMI‐ SHIFT and decreases as a function of cell height. Figure also shows that the electrolyte potential is already flat at typical cell sizes, and so the slow convergence of energies with respect to cell height arises solely from variations in rather than a slow decay of countercharge density in the continuum electrolyte …”

“…F vac is printed in VASPsol as the negative of the output flag FERMI-SHIFT and decreases as a function of cell height. Figure 1 also shows that the electrolyte potential is already flat at typical cell sizes, and so the slow convergence of energies with respect to cell height [23][24][25][26][27] arises solely from variations in F vac rather than a slow decay of countercharge density in the continuum electrolyte. [26] In a charged LPB system, the absolute value of the potential is fixed even in the presence of periodic boundary conditions due to the additional ionic screening term in the Poisson equation.…”

Valorization of Biomass‐derived Small Oxygenates: Kinetics, Mechanisms and Site Requirements of H2‐involved Hydrogenation and Deoxygenation Pathways over Heterogeneous Catalysts

“…Steinmann et al. used calculations of levulinic acid chemisorbed at a water‐Ru(0001) interface as a prototypical system to demonstrate the benefit of polarizing the quantum mechanical subsystem by an implicit solvent prior to molecular mechanics based free energy perturbation (MM‐FEP), as well as the need for dipole corrections or symmetric slabs in order to avoid artificial polarization of the metallic slab . Both the implicit solvation (polarizable continuum model, PCM) and explicit solvation (MM‐FEP) were consistent in that the bulk solvation effect is not sufficient to change the adsorption mode from bidentate to mono‐dentate, despite the fact that the carboxylic group is de‐solvated in the bidentate case.…”

Navigating Glycerol Conversion Roadmap and Heterogeneous Catalyst Selection Aided by Density Functional Theory: A Review

“…Other computational techniques, such as VASPsol (implemented in plane wave Vienna ab initio Simulation Package (VASP) code) [121], are also important tools. A comparison between implicit and explicit solvation models for surface adsorption free energies for DFT calculations was recently published by Steinmann et al [122].…”