Are DFT Methods Accurate in Mononuclear Ruthenium-Catalyzed Water Oxidation? An ab Initio Assessment

Mononuclear Ru-based water oxidation catalysts (WOCs) are an important class of WOCs for water splitting. In this work, through high-level coupled cluster calculations (CCSD(T)/CBS), we have examined a variety of density functionals for their performances in the whole catalytic cycle of water oxidation catalyzed by mononuclear Ru-based WOCs. The tested functionals cover a wide range from pure GGA and meta-GGA to hybrids and double hybrids (TPSS, OLYP, BP86, M06-L, B3LYP, PBE0, M06, M06-2X, TPSSh, CAM-B3LYP, wB97X, B2-PLYP, B2GP-PLYP). Depending on different reaction types and species in the catalytic cycle, the performances of different DFTs vary severely, whose trends are summarized in the paper. Our results indicate that using a single approximate functional to accurately model all reactions involved in the whole Ru-based WOC catalytic cycle is still a very challenging task. In the current status, PBE0 and M06 may be recommended for the whole catalytic cycle. Generally, this study provides a guide for selecting an appropriate DFT method in modeling each of the various steps in water oxidation catalyzed by Ru-based WOCs. The sensitivity of DFT and ab initio results upon the degree of basis set completeness found in this work is also worthy of attention in the future theoretical study of mononuclear Ru-based WOCs.