Sensitivity Analysis of Cluster Models for Calculating Adsorption Energies for Organic Molecules on Mineral Surfaces

Abstract

We calculated the adsorption energy for ethanol on the magnesite {10.4} surface using density functional theory (DFT) and cluster models for the mineral surface, and we quantified the errors introduced by using a finite size cluster, freezing various parts of the mineral cluster during geometry optimization and for altering the edges of the cluster. We also investigated how the adsorption energy changes for increasingly accurate density functionals, PBE, BLYP, B3LYP, and B2PLYP, also when supplemented with empirical dispersion (-D). We concluded that calculations with clusters large enough to include the surface atoms and groups binding to the adsorbate and their nearest-neighbor ions provide accurate adsorption energies, typically for MgCO3–ethanol systems, which is about 60 atoms. Using B3LYP-D and a finite size cluster of 80 atoms, we found that the adsorption energy was underestimated by 0.17 eV for adsorption from vacuum and by 0.10 eV for adsorption from solution, and we estimated a random error of ...