Therefore, theoretical as well as experimental studies have typically concentrated on the local density of states of surface metal atoms and how it is modified by their immediate environment . This change of the band width is illustrated in Figure , where otherwise the d-band center was kept constant with respect to panel a). In order to keep the number of occupied d-electrons constant, the d-band has to shift down (see Figure ). It is important to note that for metals with a completely filled d-band or a less than half filled d-band the situation is different. In the former case broadening or compression of the d-band would not cause any shift of the d-band and its center, while for early transition metals a broadening of the d-band should lead to an up-shift of the d-band center.