  Using first-principles calculations, we systematically study the potential
energy surfaces and dissociation processes of the hydrogen molecule on the
Mg(0001) surface. It is found that during the dissociative adsorption process
with the minimum energy barrier, the hydrogen molecule firstly orients
perpendicular, and then rotates to be parallel to the surface. It is also found
that the orientation of the hydrogen molecule at the transition state is
neither perpendicular nor parallel to the surface. Most importantly, we find
that the rotation causes a reduction of the calculated dissociation energy
barrier for the hydrogen molecule. The underlying electronic reasons for the
rotation of the hydrogen molecule is also discussed in our paper.
