7 Figure 6: (color online) Band structure of the (LaO) 10 (FeAs) 8 slab. The thickness of the red (shadow) lines represents in turn the orbital weight of (a) surface La 5 d + 6 s , (b) subsurface O 2 p , (c) subsurface La 5 d + 6 s , (d) next to surface Fe 3 d , and (e) inner (bulk) Fe 3 d . Figure 7: The 2D FSs of the slab (LaO) 10 (FeAs) 8 . Compare Fig. 5. The FSs from Γ outward are: two times bulk Fe 3 d , the outer one surface La 6 s ; from M outward: two times bulk Fe 3 d , the second one split in the slab, see text for explanation, two outer subsurface Fe 3 d FSs. Correspondingly dramatic are the changes in the sur- face electronic structure presented on Fig. 6. In contrast to the inner LaO triple layers the topmost La layer is to these bands (cf. Fig. 1). There the Kohn-Sham wave- function amplitude is already reduced by about an order of magnitude. As seen in panel (b) of Fig. 4, there are three surface bands forming FS hole cylinders around Γ. Two of them have radii only slightly larger than the bulk FSs but one has a radius about twice as large. It results from a band of Fe 3 d$_{z}$ 2 orbital character which is completely occupied Table II says that the surface relaxation of an LaO sur- face triple layer is dramatic. While the geometry of the La tetrahedra changes little, the O sites move outward by about 0.3 ˚ A. The charge per unit cell of the O layer changes by 0.3 electron charges compared to the bulk while in the case of an FeAs surface triple layer the cor- responding change of the Fe charge was only within 0.02 electron charges. The subsurface La layer charge still changes by 0.2 electron charges which induces even some change in the next FeAs triple layer and only beneath that a near bulk situation is found. In contrast, only the charge of the topmost As atom layer differs notice- ably from the bulk value in the case of the As terminated surface. in the bulk. Since all conduction bands of these materi- als are Fe-As antibonding, 4 it is shifted up by the surface compression of the As tetrahedra in z -direction strength- ening the FeAs covalency of the 3 d$_{z}$ 2 orbital with the As 4 p orbitals around Γ ( k = 0). There are again two electron cylinders around M , but they are tiny having radii about three times smaller than the corresponding bulk FSs. The surface states are essen- tially only formed by Fe 3 d -orbitals of the surface triple layer. They have very small amplitudes everywhere else. D. The La terminated surface