Chlorine and HCl are adsorbed dissociatively on the Cu(100) surface. The exposure of Cl to copper surfaces lead to the formation of CuCl, which develops later to CuCl2. In this paper, we show, using density functional theory calculations, that the presence of adsorbed water on a Cl-Cu(100) modified surface leads to a low energy pathway to a direct CuCl2 formation.(C) 2017 Elsevier B.V. All rights reserved.