Density-functional theory (DFT) and direct molecular dynamics (MD) calculations have been performed to characterize the hydrogen underpotential deposition (UPD) on platinum electrode. Hydrogen over platinum (111) surface in aqueous phase exhibits two adsorption sites with small absorption energies and a high mobility. Under high-coverage conditions, adsorbed H atoms rearrange to form honeycomb-like structures with 1:1 coverage of electrode surface atoms, the strong H-H interaction immobilizing the adsorbed species. The mechanism for hydrogen evolution at high coverage is examined and the activation barriers for hydrogen evolution and oxidation under different electrode potential conditions are determined. Vibrational frequency analysis is carried out for the adsorbed species at different degrees of coverage and electrode potential to compare with experiment.