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A difficulty in the generation and testing of intermolecular interaction potentials has to do with the effects of vibrational motion. For weak modes, the vibrational excursions can be and often are sizable. So, not only do model potentials have to be suitable over a considerable range, they must also be accurate with respect to experimental data that reflects the vibrational averaging (i.e., on-average structures). In this report, we present an explicit interaction potential for the water dimer that has been devised with a systematic treatment of vibrational effects by means of quantum Monte Carlo treatment of the vibrational ground state. The model potential uses intrinsic electrical properties of isolated water and four empirical parameters. An iterative procedure based on matching measured on-average rotational constants to calculated values was used to select the atom–atom Lennard-Jones (empirical) parameters. We present the results of these calculations, including zero point vibrational energies and on-average separations.

Molecular nitrogen is nonpolar and has a relatively small dipole polarizability, making for particularly weak intermolecular interactions. The characterization of nitrogen interaction potentials is important for simulations, and in this study, we present an explicit model potential built around electrical interaction elements. The polarizability of nitrogen is found to be a small contributor to interaction in pure N2 clusters but is relatively more important in mixed clusters. The generally weak attachment of N2 to other species makes for a tendency to have attractive interactions at more than one site of the partner. In the complex with water, for instance, nitrogen bonds at the oxygen and at the protons, and the potential surface exhibits low barriers for several interconversion processes. Likewise, nitrogen is predicted to attach in two ways to acetylene.

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