In this quantum chemical study it is shown that a hydrogen atom chemisorbs weakly to a surface carbon atom on hydrogen-terminated diamond (111) if there is a bond-inserted hydrogen atom, or a substitutional B atom, in the near-subsurface region. It is found that reductive hydronium ion discharge in the Volmer-Heyrovsky mechanism to form H-2 is activated over these adsorbed H atoms compared to the strongly bonded H that terminates the surface. Activation energies for H-2 generation are calculated for these systems as functions of electrode potential between 0 and -3.2 V, using constrained variation theory. (c) 2005 The Electrochemical Society.