First-principles study of water adsorption monolayer on Pt(111): Adsorption energy and second-order nonlinear susceptibility

To establish first-principles calculation accuracy and methodology for a metal/water interface, we investigated the H2O adsorption monolayer on a Pt(111) surface and developed a density functional theory (DFT)-based method for analyzing the second-order nonlinear susceptibility chi((2))(zzz). The H2O adsorption structures of root 3 x root 3 with H-up/H-down models, root 39 x root 39 with a six-member ring model, root 39 x root 39, and root 37 x root 37 with five- and seven-member ring models were considered by applying DFT calculations. The last two structures were found to be the most stable, and their heats of adsorption, including zero-point vibration correction, were in good agreement with the activation energy of desorption in the experiment with the vdW-DF2-B86R functional. Fur-thermore, by employing the density functional perturbation formulation combined with the effective-screening medium (ESM) + a finite electric field approach, we obtained the second-order nonlinear susceptibility chi((2))(zzz) of the root 39 and root 37 5-7-ring structures. The negative and positive signs of Im chi((2))(zzz) in the high (3000-3500 cm(-1)) and low (600-2400 cm(-1)) frequency regions, respectively, were obtained. The spectra show negative double peaks in the high-frequency region, consistent with those obtained by heterodyne-detected sum frequency generation spectroscopy. This study provides a fundamental basis for improving our understanding of H2O adsorption on metal surfaces.