Constrained molecular dynamics simulations are carried out to compare the adsorption/incorporation mechanism of two (nonionizable) hydrogen halide acid molecules HF and HCl on/in ice at typical stratospheric temperatures (190 and 235 K). From the determination of the free energy profiles, it is shown that the free energy barrier to molecular HF incorporation is higher than that for molecular HCl. This difference is interpreted as resulting from the formation of two strong H bonds between HF and its water neighbors, while a single, more flexible, H bond with water favors the penetration for HCl. (C) 2003 American Institute of Physics.