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.).