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 surfa ce and performing a complete dynamics on that surface have been examined fo r the Ar-HF van der Waals complex. We have employed a symmetry-adapted pert urbation theory potential including the dependence on the H-F internuclear distance r. This potential was used to obtain a reference rovibrational spe ctrum of Ar-HF from the complete three-dimensional dynamics calculations. F rom the three-dimensional surface we have generated several two-dimensional potentials: the vibrationally averaged potential and the potentials obtain ed by fixing r at its equilibrium value r(e) and at the vibrationally avera ged 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 c orresponding to r = r(e). The spectrum closest to the reference one is give n 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 stab ilizing effect of the monomer deformation upon dimer formation, has also be en 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 energ y with an error of the order of O(V-3) has been given and numerically teste d. (C) 2000 American Institute of Physics. [S0021-9606(00)30432-9].