Using first-principles calculations, we calculated the atomic structures and surface energies of molybdenum surfaces in the ͑100͒, ͑110͒, ͑111͒, and ͑211͒ orientations. The equilibrium crystal shape of molybdenum is then found using the Wulff construction. We find that all four orientations appear on the Wulff plot and hence they are stable. ͓S0163-1829͑98͒06803-9͔

The faceting of Pd/W͑111͒ to form nanoscale three-sided pyramids with ͕011͖ facets has been observed using a scanning tunneling microscope ͑STM͒. In contrast to the well-documented ͕112͖ faceting, which can transform the entire Pd/W͑111͒ surface to a morphology composed completely of pyramidal ͕112͖ facets, the ͕011͖ faceting of Pd/W͑111͒ occurs in coexistence with ͕112͖ faceting upon prolonged annealing. While ͕112͖ facets are found to grow in size ͑ranging from ϳ3 to 15 nm͒ with increasing annealing temperatures, the dimensions of ͕011͖ facets appear to be limited to ϳ2.5 nm. The coexistence of two types of pyramidal facets is consistent with energetic considerations based on first-principles calculations. The difference in their typical sizes is, on the other hand, interpreted as an evidence for kinetic factors in the faceting processes.