An explicit charging approach is presented to account for constant potential in atomic level electrochemistry modeling. Explicit charging allows for the calculation of electrochemical reaction barriers, while also reproducing the results from the computation hydrogen electrode (CHE) approach when the reaction modeled with CHE is at constant potential. Explicit charging is used to obtain activation and adsorption free energies for proton adsorption (the Volmer reaction) from a static water bilayer to the Cu(111) surface. The proton adsorption becomes spontaneous at around 0 V vs RHE and the adsorption free energy changes by around 1 eV per V (similar to the CHE result). The activation free energy changes by around 0.5 eV per V, but is still prohibitively large at 0 V vs RHE. This is consistent with experiments, where hydrogen evolution is only observed below -0.4 V vs RHE.