In order to deconvolute the effect of each descriptor, we consider the previously synthesized complexes built with bis(diphenylphosphino) methane (dppm), bis(diphenylphosphino) benzene (dppb), bis(diphenylphosphino) 9,9'dimethylxanthene (dppx), and bipyridine (bpy) ligands (given in Figure ), as well as two types of simplified systems not available experimentally: 1) analogs of the full phosphorus ligands without phenyl groups (referred to as pm, pb, and px respectively) and of the bpy ligand without the pyridine rings (to form a bis(methyleneamino) ethene ligand, referred to as mae), shown in Figure and) minimal model complexes where we reduce the ligand to its essential part: the element directly bound to the metal (N or P) with two possible coordinations (c 3 or c 4 ), shown in Figure . These models allow us to isolate individual features that can affect the reactivity, and to vary them in a systematic way. Initial geometries for the previously synthesized complexes were taken from experimental structural data from single-crystal X-ray diffraction (XRD) analysis available for Cp * Rh(dppm), Cp * Rh(dppx), and Cp * Rh(bpy). To determine the reactivity of each ligand framework, we could compute the reaction energy (∆E) of each step in the catalytic cycle in Figure for each complex; however, this presents a problem in defining the energy for certain species, such as the solvated proton and an individual acetonitrile molecule in solution. While empirical estimates can be obtained for some of these values, they are challenging to compute by purely ab initio means. Another approach, which we have chosen here, is to calculate the change in reaction energy (∆∆E R ) relative to a reference choice of ligand. While this method does not explicitly predict individual reaction energies, it does allow for a relative ordering of the reaction energies for different complexes undergoing the same reaction step. It also has the distinct advantage that only the energies of the complexes are needed, because the energies of common species across reactions cancel out. The bpy complex is a convenient reference as it has been shown to be an effective hydrogen evolution catalyst, 4 making the computed relative reactivity a proxy for catalytic effectiveness. With the Rh(I)bpy complex as reference, ∆∆E R1 X values for Reaction 1 can be calculated as follows: