On the optimal choice of monomer geometry in calculations of intermolecular interaction energies: Rovibrational spectrum of Ar-HF from two- and three-dimensional potentials

Alternatives to using a full-dimensional interaction-potential energy surface and performing a complete dynamics on that surface have been examined for the Ar-HF van der Waals complex. We have employed a symmetry-adapted perturbation theory potential including the dependence on the H-F internuclear distance r. This potential was used to obtain a reference rovibrational spectrum of Ar-HF from the complete three-dimensional dynamics calculations. From the three-dimensional surface we have generated several two-dimensional potentials: the vibrationally averaged potential and the potentials obtained by fixing r at its equilibrium value r(e) and at the vibrationally averaged distances [r(-2)](-1/2), [r], [r(2)](1/2), and [r(3)](1/3). For all two-dimensional potentials obtained in this way the rovibrational spectra have been computed and compared with the reference spectrum. We have found that the potential obtained by setting r = [r] performs much better than that corresponding to r = r(e). The spectrum closest to the reference one is given by the vibrationally averaged potential. Of all potentials computed for a fixed r, the potential corresponding to r = [r(3)](1/3) performs best. The role of the so-called relaxation energy, computed often to assess the stabilizing effect of the monomer deformation upon dimer formation, has also been investigated. It has been found that this energy is of the order O(V-2), where V is the interaction potential, and is expected to be negligible for molecules as rigid as HF. A simple formula estimating the relaxation energy with an error of the order of O(V-3) has been given and numerically tested. (C) 2000 American Institute of Physics. [S0021-9606(00)30432-9].