Ab initio simulation of 'liquid' water on a Pd surface

We report on the application of the Car-Parrinello molecular dynamics method (CPMD) to the interaction of water ensembles with Pd(100) model surfaces. Periodic Pd(50)O(8) and Pd(18)O(2) slabs covered with nine and 18 water molecules, respectively, were employed to analyze the balance of adsorbate-adsorbate and adsorbate-surface interactions as a function of water ensemble size, in the limiting cases of a clean surface and high surface coverage with atomic oxygen. Our simulations suggest that 'liquid' water molecules are distributed, for the most part, randomly on Pd(100) at 300 K, with a characteristic intermolecular distance distribution in distinct regions of the water/metal interface. The structure of low-mobility local water clusters which stay close to the surface resembles that of earlier proposals based on thin him experiments. In addition, our results corroborate the experimental finding that a high surface coverage with oxygen reduces the water-Pd interaction compared to that on the clean solid.