The trapping sites of HArF and HKrF in crystalline Ar and Kr are investigated computationally. Ab initio calculations are used to evaluate interactions between the rare gas containing molecule and a single rare gas atom. Molecular mechanics and molecular dynamics are used to study the properties of HArF and HKrF in rare gas crystals. Three different trapping configurations have been found for both molecules. The lowest-energy site is a double-substitutional (DS) configuration and the second lowest energy site is a single-substitutional (SS) one. The DS site can be interpreted to involve a 1:1 Rg.HRgF complex (Rg=Ar, Kr). The energy difference between these sites is 10.4 and 9.8 kJ/mol for HArF and HKrF, respectively. All the computational evidence shows that the experimentally observed stable site of HArF [J. Am. Chem. Soc. 123, 8610 (2001)] corresponds to a DS site and the unstable site corresponds to a SS site. Relaxation of the SS site to the DS site involves the motion of a vacancy in the lattice and this suggests that HArF and HKrF can be used to study the dynamics of vacancy motion in rare gas solids. (C) 2003 American Institute of Physics.