An ab initio intermolecular potential energy surface for the F2 dimer

Two analytical representations for the potential energy surface of the F-2 dimer were constructed on the basis of ab initio calculations up to the fourth-order of Moller-Plesset (MP) perturbation theory. The best estimate of the complete basis set limit of interaction energy was derived for analysis of basis set incompleteness errors. At the MP4/aug-cc-pVTZ level of theory, the most stable structure of the dimer was obtained at R = 6.82 au, theta(a) = 12.9 degrees, theta(b) = 76.0 degrees, and phi = 180 degrees, with a well depth of 716 mu E-h. Two other minima were found for canted and X-shaped configurations with potential energies around -596 and -629 mu E-h, respectively. Hexadecapole moments of monomers play an important role in the anisotropy of interaction energy that is highly R-dependent at intermediate intermolecular distances. The quality of potentials was tested by computing values of the second virial coefficient. The fitted MP4 potential has a more reasonable agreement with experimental values.