Potential energy surfaces of van der Waals complexes of water and hydrogen halides modeled using distributed multipoles

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Abstract
Analytic first and second derivatives of the energy have been calculated for intermolecular potentials based upon distributed multipoles (to describe the electrostatics) and atom–atom Lennard‐Jones terms (to describe dispersion and repulsion). This enables us to employ eigenvector‐following to determine minima, transition states, and rearrangement mechanisms for a variety of van der Waals complexes. Where previous ab initio studies are available the agreement is usually satisfactory, but in some cases we find that a geometry corresponding to an ab initio minimum is a transition state with the model potential, or vice versa. Even in such cases the stationary points we identify will be useful in guiding more accurate calculations. The rearrangement mechanisms should be of particular interest in determining effective molecular symmetry groups and splitting patterns due to tunneling when low rearrangement barriers are present.