  While the small sticking coefficient for molecular hydrogen on the Si(001)
surface apparently requires a large energy barrier of adsorption, no such
barrier is observed in desorption experiments. We have calculated the
potential-energy surface of an H_2 molecule in front of a Si(001) surface. If
we relax the Si substrate, we find an optimum desorption path with a low
(\lesssim 0.3 eV) adsorption energy barrier. While molecules impinging on the
surface will mostly be reflected at the larger barrier of some
frozen-substrate, molecules adsorbed on the surface can desorb along the
low-barrier path.
