Systematic implementations of density functional calculations of magnetic materials, based on atomic orbitals basis sets, are scarce. We have implemented in one such code the ability to compute non-collinear arrangements of the spin moments in the GGA approximation, including spiral structures. We have also made a thorough study of the degree of accuracy of the energy with the size of the basis and the extent of the orbitals. We have tested our results for the different phases of bulk iron as well as for small clusters of this element. We specifically show how the relative stability of the different competing states changes with the degree of completeness of the basis, and present the minimal set which provides reliable results.