The major structural difference between P411-PFA and the P-4 variant used previously for selective amination is the perturbation of the helix directly over the binding site. In P4110PFA, a residue mutation induces a flip in orientation resulting in a site of increased steric hindrance. This artifact hinders the distal portion of the binding cavity, shown by a decrease in distVCAVITY from 260Å 3 to 237Å 3 (Figure ). This distal hindrance around the porphyrin may influence observed selectivity by restricting the approach of the N-phenyl-morpholine substrate. CONCLUSION Reactive cavities are difficult to sterically parametrize in mechanistically meaningful ways using traditional molecular descriptors. A free, open-source Python package, SMARTpy, is introduced to compute SMART molecular descriptors that have been disclosed in application to dirhodium(II) selectivity. SMART descriptors provide information about the steric environment within a reactive cavity from the perspective of a bound or docked substrate. Though designed for dirhodium catalysts, we envision a broad scope of applicability to diverse systems.