DFT calculations identified three main pathways involving different reaction intermediates in the formation of CO and HCOO -(Fig. ). Two intermediates arising from the first protoncoupled electron transfer produce CO (*OCOH and *OCOH'), and the other intermediate produces HCOO -(*COOH) (Fig. ). We performed calculations on each CO2 reduction and hydrogen evolution pathway with and without the addition of SPy (Figs. ). There was no alteration to the free energy profile for *H, showing that the molecule does not stabilise hydrogen on the surface; this also supports the experimental results which showed an unaltered FEH 2 with Cu-SPy (Fig. ). Interestingly, the formate pathway showed only minimal free energy changes, as did one of the CO generating pathways (involving *OCOH') (Figs. ). However, the other CO pathway (involving *OCOH) showed significant increases in the free energies of both the *CO2 -and *OCOH intermediates in the presence of SPy, thereby blocking this CO2R pathway (Fig. , Fig. ). Electron density difference plots identified a destabilising interaction between the N atom of the SPy and the surface-bound intermediate, accounting for this increase in free energy (Fig. ). These results show that one of the CO production pathways is thermodynamically inhibited by the presence of SPy. This molecular inhibition mechanism agrees with the described experimental observations that HCOO -production comes at the expense of CO and C2H4 formation. Thus, by preventing CO generation, the surface-bound SPy directs selectivity towards formate.