The transition states of the elementary reactions for the dissociation of methane on the ruthenium (0001) surface have been investigated with DFT periodic calculations and the nudged elastic band method (NEB) for 25% coverages. The calculated barriers are 85 kJ mol-1 for methane decomposition, 49 kJ mol-1 for methyl decomposition, and 16 kJ mol-1 for methylene decomposition, respectively. The decomposition of CHads requires the highest activation energy from the series with 108 kJ mol-1. Discussion concerning chemical bonding aspects of the transition states structures is provided for.

The results of ab initio molecular dynamics simulations of liquid water and liquid water–vapor interface using the Perdew-Wang 91 (PW91) exchange-correlation functional are presented. The structural and transport properties of liquid water are comparable to the previous results using Becke-Lee-Yang-Parr (BLYP) functional and experimental data. The shape and the position of the first peak in the oxygen–oxygen radial distribution function is in good agreement with the most recent neutron diffraction data. The ab initio molecular dynamics simulation of liquid water–vapor interface, which is the first of its kind, suggests a preferred orientation of the surface water dipole towards the bulk region.

We present a dynamic Monte-Carlo model involving lateral interactions and different adsorption sites ͑top, fcc and hcp͒. Using this model in combination with kinetic parameters from UHV experiments and lateral interactions derived from DFT calculations we have reproduced the ordering behavior of NO on Rh͑111͒ during adsorption and the temperature programmed desorption ͑TPD͒ of NO from Rh͑111͒ under UHV conditions. The formation of c͑4ϫ2͒-2NO domains at 0.50 ML coverage is shown to depend strongly on the next-next-nearest-neighbor repulsion between the NO adsorbates in our model. The formation of the ͑2ϫ2͒-3NO structure at higher coverage follows from the avoidance of the strong next-nearest-neighbor repulsion in favor of the occupation of the top sites. A single-site model was able to reproduce the experimental TPD, but the lateral interactions were at odds with the values of the DFT calculations. A three-site model resolved this problem. It was found that all NO dissociates during TPD for initial coverages of NO below 0.20 ML. The nitrogen atoms recombine at higher temperatures. For NO coverages larger than 0.20 ML, 0.20 ML NO dissociates while the rest desorbs. This is due to a lack of accessible sites on the surface, i.e., sites where a molecule can bind without experiencing large repulsions with neighboring adsorbates. For NO coverages above 0.20 ML, the dissociation of NO causes a segregation into separate NO and NϩO islands. The dissociation causes the surface to be filled with adsorbates, and the adsorbates are therefore pushed closer together. NO on one hand can easily be compressed into islands of 0.50 ML coverage, because there is no large next-next-nearest-neighbor repulsion. NϩO on the other hand form islands with a lower coverage ͑0.30-0.35 ML͒ due to the considerable next-next-nearest-neighbor repulsion. Top bound NO ͑above 0.50 ML initial coverage͒ does not dissociate during TPD. It desorbs in a separate peak at 380 K.

The adsorption of ammonia on the two low index ͑111͒ and ͑100͒ surfaces of rhodium has been studied by periodic calculations with density functional theory and compared to experimental results. The geometries of the adsorbates and the surfaces are completely optimized. For both surfaces the top site is found to be the most stable while the adsorption energy of ammonia is 8-10 kJ•mol Ϫ1 larger on the ͑100͒ surface. The presence of steps on the ͑100͒ surface has a minor effect on the heat of adsorption. The theoretical predictions of the adsorption energies and the changes in work function by NH 3 are in good agreement with experimental data. Moreover the prediction of the ontop adsorption as well as the weak interactions between the adsorbates is confirmed. The broadening of the temperature programmed desorption spectra and the two desorption peaks for the first adlayer are mainly due to an entropy effect which affects the preexponential factor of the desorption rate constant.

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The interaction of CO with the Ru(0001)(1ϫ1)H surface has been studied by density functional theory ͑DFT͒ periodic calculations and molecular beam techniques. The hydrogen (1ϫ1) phase induces an activation barrier for CO adsorption with a minimum barrier height of 25 kJ mol Ϫ1 . The barrier originates from the initial repulsive interaction between the CO-4 and the Ru-d 3z 2 -r 2 orbitals. Coadsorbed H also reduces the CO adsorption energy considerably and enhances the site preference of CO. On a Ru͑0001͒͑1ϫ1͒H surface, CO adsorbs exclusively on the atop position.

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