First principles molecular dynamics has been used to simulate the interface between a defective Pt surface that contains a pit with coadsorbed OH and liquid water under a bias. Water dissociation was not observed even under a strong bias (corresponding to a Pt surface charge of sigma = 14 mu C/cm(2)) although it was observed on a flat surface with less surface charge. The OHad is located at a top site of the flat surface but it is at a bridge site of the pitted surface. Surface morphology and hydrogen bonding, therefore, have a significant impact on water dissociation. The OHad promotes water dissociation on the flat surface because it makes a hydrogen bond with a water molecule adsorbed on the Pt top site; however, on the pitted surface, water dissociation is prohibited because of an unfavorable formation of a hydrogen bond between the OHad on the bridge site and the H2O on the top site. Our electronic structure calculation shows an unsymmetrical electron distribution between the two OH bonds of the water molecule, in which the OH bond with fewer electrons (opposite side of the hydrogen bond with OHad) is dissociated to produce H3O+ with another water molecule. (C) 2012 Elsevier B.V. All rights reserved.