  Hierarchically combining semi-empirical methods and first-principles
calculations we gain a novel and noteworthy picture of the molecular-level
mechanisms that govern the water-gas-shift (WGS) and reverse water-gas-shift
(r-WGS) reactions on Rh catalysts. Central to this picture is that the WGS and
r-WGS follow two different dominant reaction mechanisms: WGS proceeds according
to a carboxyl (COOH) mechanism, whereas r-WGS proceeds according to a redox
(CO2 {\to} CO + O) mechanism. The obtained results furthermore underscore the
danger of common first-principles analyses that focus on a priori selected
dominant paths. Not restricted to such bias, our herein proposed hierarchical
approach thus constitutes a promising avenue to properly transport and
incorporate the ab initio predictive-quality to a new level of system
complexity.
