The pH effect in hydrogen evolution is a consequence of creating a solvated hydroxide ion

<jats:p>The hydrogen evolution reaction is orders of magnitude faster in acid than in base on the same electrode, a pH effect that first-principles calculations have not quantitatively reproduced. We trace this failure to the treatment of the ions: standard density functionals with standard solvation treatments undersolvate hydroxide–the ionic product of alkaline hydrogen evolution–by more than 1 eV against experiment and coupled cluster benchmarks, an error that tracks the functional’s self-interaction error. A screened hybrid functional with reduced self-interaction error yields Au(111) Volmer barriers whose acidalkaline difference falls within the measured range of onset-potential separations, whereas semi-local functionals predict zero to three times the measured value. Quantitative computational electrocatalysis therefore demands functionals benchmarked jointly against ion solvation thermochemistry and molecule-surface energetics.</jats:p>