ORIENTATION DEPENDENCE OF THE HYDROGEN MOLECULES INTERACTION WITH RH(001)

First-principles calculational results, that conflict with conventional wisdom, are reported for the orientation dependence of the energy of H-2, a few bohrs above a Rh(001) surface. In geometric configurations near the saddle point where H-Rh bonds begin to form, the favored molecular axis orientation is along a <100> direction, corresponding to the attraction of each H atom to a twofold bridge site. This is true even though the final destinations of the H atoms are fourfold hollows, and even though the <100> orientation corresponds to relatively low symmetry, i.e., C(s) rather than C2v.