We present a general formalism for polarizable electrostatics based on fluctuating bond-charge increments and polarizable dipoles and its application to a five-site model for water. The parametrization is based largely on quantum-chemical calculations and should be easily transferable to other molecules. To examine basis-set effects we parametrized two models from two sets of quantum calculations, using the aug-cc-pVTZ and aug-cc-pVQZ basis sets. We computed several gas-phase and condensed-phase properties and compared with experiment or ab initio calculations as available. The models are quite similar and give condensed-phase properties at ambient conditions that are in reasonable accord with experiment, but evince errors consistent with a liquid-state dipole moment that is slightly too large. The model fit to the aug-cc-pVTZ basis set has a smaller liquid-phase dipole moment and thus gives a somewhat better description of liquid water at ambient conditions. This model also performs well away from room temperature, deviating less than 2% from the experimental density from 0 to 100°C, and showing good agreement with experimental radial distribution functions, although the temperature of maximum density (ϳ20°C͒ is slightly too high and the model somewhat underpredicts the persistence of the hydrogen-bond network at elevated temperatures.

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Grand canonical and canonical ensemble Monte Carlo computer simulations of the adsorption of N2 on the (110) face of rutile at 77 K are reported. A novel ab initio adsorbate−adsorbent interaction potential is employed in conjunction with the X1 nitrogen−nitrogen potential to investigate the adsorption mechanism. It is demonstrated that at low pressures (1 Torr and below) the Ti adsorption sites within the depressed rows of oxides on the rutile (110) face (denoted by A) are completely occupied by nitrogen molecules in end-on orientations with slight alternating tilts perpendicular to the row axis that are produced by repulsive lateral interactions. At higher pressures, adsorption on rows of exposed oxides (denoted by B) commences, typically with a side-on orientation of the N2 molecules. The calculated isotherm of adsorption exhibits type II behavior according to the Brunauer−Deming−Deming−Teller classification, in agreement with experimental findings. Although the experimental isotherms are often evaluated using the Brunauer−Emmett−Teller adsorption model, our simulations indicate that the assumptions of this model are not fulfilled. The implications of these discrepancies and their influence on surface area determinations are discussed.

A full five-dimensional potential energy surface for the interaction of nitrogen molecules with the (110) surface plane of TiO2 (rutile) is generated. In a first step, ab initio SCF cluster calculations are performed for various adsorption geometries of N2 above the TiO2(110) surface, which is described by different stoichiometric clusters, ranging in composition from Ti7O14 to Ti13O26, embedded in extended point charge fields. The N-N distance is fixed to the experimental equilibrium bond length 1.098 Å. In a second step, a simple analytic form for the interaction potential is developed which contains the electrostatic interaction between the charge distribution of N2 and the electric field above the surface, the polarization of the N2 molecule by this field, and the Pauli repulsion between N2 and the surface. By fitting the five parameters in the analytic expression (quadrupole moment and the two polarizability components of N2, repulsive Lennard-Jones parameters between N and the O and Ti atoms of the surface) to the calculated ab initio interaction energies, one can represent the full five-dimensional interaction potential with a mean error of about 3 kcal/mol. The global minimum of the interaction potential is found for the vertical end-on adsorption of N2 on a coordinately unsaturated row A titanium atom; it has an adsorption energy of -46 kJ/mol and a Ti-N distance of 2.39 Å. The side-on adsorption of N2 on the row B oxygen atoms with the N-N axis perpendicular to the row B direction is also slightly attractive with a small adsorption energy of -5.5 kJ/mol. † This paper was initially intended for the special issue of Langmuir devoted to the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids.

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