The optimized energies for all the structures are provided in Table , 4. Through these DFT computations we seek: (i) to understand which reaction intermediates are involved in the C-S coupling step; (ii) to calculate the reaction energy barrier of the C-S coupling step; and (iii) to specify the best performing facet for the C-S bond formation. The optimal path is shown with thick black arrows on Fig. . The energy diagram comparing different C-S coupling steps is shown on Fig. . Coupling through 𝑆𝑂 is more favourable than 𝐻𝑆𝑂 , regardless of which intermediates are involved in the C-S coupling step and no matter what the copper surface is. Furthermore, due to the high adsorption energy of 𝑆𝑂 * on (100), surface coupling is deemed less likely than the nucleophilic attack of the species from the electrolyte. The optimal coupling step is found to be through *CHOH. The energy barrier of the C-S coupling step is 0.74 eV and is the rate determining step (RDS). Coupling through *COH has a large energy barrier of 1.37 eV. Surprisingly, coupling through *CH2OH has an energy barrier of 0.66 eV, which is lower than *CHOH coupling. However, protonating the sulfur of SO3CH2OH has a barrier of 0.82 eV, making this path unfavourable. Comparing the energy barrier of the RDS to typical RDS's of CO2RR, it is reasonable to observe that the maximal FE obtained in this study is ~10% since most thermodynamic energy barrier for the CO2RR are lower than 0.74 eV. Lowering the energy barrier of the *CHOH coupling step is key to further increasing the FE in future studies. The same methodology and calculations are applied to the (110) facet since it appears to have the lowest 𝑆𝑂 * adsorption energy. In other words, the (110) facet has the strongest affinity for 𝑆𝑂 * , making it an interesting catalyst. Figure shows the energy diagram of the most favourable path on both (100) and ( ). The most promising coupling path on (110) appears to be consistent with *CHOH coupling being the most favourable again. The coupling energy barrier, however, is only at 0.11 eV. The largest subsequent protonation energy barrier is the protonation of sulfur in SO3CHOH to HSO3CHOH, at 0.29 eV as can be seen in Fig. , 29. Thus, the RDS remains *CO hydrogenation to *CHO, a typical RDS for the CO2RR. The DFT results show that (110) should be superior to (100) when it comes to C-S bond formation as seen in Figure however our experiments do not show that. This could be due to changes in size, surface reconstruction and other unaccounted-for effects. We encourage future studies to explore the Cu (110) facet further.