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The equilibrium structures, binding energies and infrared spectra were computed using the ab initio molecular orbital theory for the weakly bonded van der Waals complex H 2 O·F 2 . The most stable state was found to possess an effectively planar (as defined by Cooke et al. (Angew. Chem., Int. Ed. Engl., 36 (1997) 129)) C s structure with the monomers bonded through the Lewis acid-base type interaction. The zero-point HHO inverting motion could overcome the low barrier of a planar configuration and as a result, produces the spectroscopic features observed in the previous microwave measurement. Several intense far infrared vibrational modes were predicted. A considerable uncertainty, arising from the ill-represented harmonic force field presumed by the ab initio theory, was identified for the vibrational analysis. Finally, the quartic centrifugal distortion constants were estimated.