Surface calculations are definitely very subtle, and you should be rather careful if you want to do such calculations. Before starting read the section 8 with great care and understand the basic outlines of this section. In the following chapters we will explain the typical steps involved in a surface calculation. Even if you follow all these steps difficulties might come up. So whenever you get physical meaningless results first think about your possible mistakes (see section 8): i.e. are your FFT-meshes sufficient, have you used enough k-points, is your calculation converged correctly, are your positions correct, - in general - are the parameters in the INCAR, POSCAR, KPOINTS file and the param.inc file chosen correctly. Also mind that an error in an early step of the calculation might result in serious errors for all successive calculations. For instance an error of in the lattice constant might result in an error of up to in the calculation of the surface relaxation. So it is a good idea to spend more time in the first few steps (bulk calculation, determining the necessary size of the FFT grids, k-points etc.).