Underpotential deposition of copper on Au(111) is analyzed in the light of first-principles calculations. The existence of a 1x1 Cu monolayer on Au(111) at underpotentials can be only understood taking into account the free energy excess delivered by the adsorption of sulfate anions. While a naked copper pseudomorphic monolayer results thermodynamically less stable than bulk copper, accounting for the interaction with sulfate ions delivers an underpotential shift which is comparable with the experimental value.

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