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overlap across the slit. These slab structures were now relaxed using the forces on atom sites computed with the FPLO code. The obtained surface relaxation does prac- tically not change, if the spacing between (LaO) 4 (FeAs) 6 slabs is increased from 13 ˚ A to 30 ˚ A, and it stays within 0.02 ˚ A, about twice the numerical accuracy of the calcu- lation, between the (LaO) 4 (FeAs) 6 and (LaO) 8 (FeAs) 10 slabs. 

As already in the cleavage process, the La terminated surface is more relaxing than the As terminated surface. Moreover, it turns out that the (LaO) 6 (FeAs) 4 slab is not thick enough in this case. The surface triple layer dopes more charge into the neighboring FeAs triple layer rais- ing there the Fermi level against its bulk value. Besides the surface potential step, there are two more essential influences on the electronic structure of a slab: structure relaxation and a chemical potential shift due to surface states. The latter is not operative on the surface of a bulk crystal where the chemical potential (Fermi level) is fixed to its bulk value. For this to be approximately true the (LaO) 6 (FeAs) 4 slab turns out to be not thick enough. Therefore, we concentrate on a (LaO) 10 (FeAs) 8 slab. 

Table II shows the relaxed interlayer distances in com- parison with their bulk values. For the As terminated slabs, essentially only the topmost triple layer relaxes by 

reducing the As-Fe layer distances by about 0.05 ˚ A while all other distances inside triple layers stay at their bulk values within 0.01 ˚ A accuracy. (Again, the bond between LaO and FeAs triple layers due to its weakness may relax somewhat more, 0.03 ˚ A in one case.) For the La termi- nated surface the relaxation is much more dramatic, it amounts to an about 0.3 ˚ A movement of the O posi- tion outwards from the center of the La tetrahedra. This continues into changes of about 0.03 ˚ A of the Fe-As layer distances and even larger changes of the other inside the slabs. 

As already mentioned in Section I, also the layer charges were compared with their bulk values in order to assess the quality of approaching the situation of a bulk crystal surface. As typical results, in Table III the GGA layer charges as obtained by FPLO are dis- played for a bulk crystal, a cleaving crystal as described in subsection A, several As terminated slabs and for (LaO) 10 (FeAs) 8 . As one can see, the charge per unit cell of the central triple layer of the slabs deviates less than 0.01 electron charges from the bulk value, while for the unsymmetric slab of the cleavage situation the deviations are larger due to the additional electric field produced by the non-zero gross dipole density. 

