Eq \ref{eqn:procs} is parameterized using OPBE/6-31G(d,p)//PM6 calculations so we compare ProCS15 against full OPBE/6-31G(d,p)//PM6-D3H+ calculations on Ubiquitin (1UBQ) and GB3 (2OED) to test for errors introduced by the inherent additivity assumptions and the structural simplifications in the model systems used for the DFT calculations. We use PM6-D3H+ for the geometry optimization, rather than PM6, to get a better description of hydrogen-bonding and other intermolecular interactions. However, bond lengths and angles, and their effect on chemical shifts, will be very virtually identical to PM6. The results are summarized in Table \ref{table:vsqm}. The first row, marked “all”, summarizes results for ProCS15 if all but the last term of Eq \ref{eqn:procs} are included. The last term corrects for the explicit solvent effects and thus not relevant when comparing to DFT calculations.