We have simulated a semi-infinite system of cobalt fcc and evaluated the value of the work function for different crystal faces, (100) and (111). The obtained results are presented in Tab. 6.1. As we can see, the value of the WF of the face is slightly larger than that corresponding to the face , the increment of the work function of the cobalt's face being only about . Smoluchowski et. al.  have studied the case of a simple cubic lattice in simple metals. They obtained the result that the lowest work function corresponds to the least densely packed face among those considered. Our results presented in Tab. 6.1 show analogous values but slightly lower for the (111) case.