  We study the anisotropy energy in a planar model for the Cu-O layers of the
La2CuO4 investigated in previous works. The Tight-Binding (TB) potential of the
model was extended out of the validity region with the purpose of incorporate
it in the potential. Next, the spin-orbit operator was considered in the
Hartree Fock solution of the full HF problem. It follows that in spite of the
fact that the extended potential possesses the square symmetry of the
crystalline structure, the anisotropy energy vanishes in its purely 2D
formulation. The result indicates that for the prediction by the model of the
observed non vanishing magnetic anisotropy at zero doping and temperature in
La2CuO4, its formulation requires of a 3D representation of its Wannier
orbitals and the use of d like orbitals. The consideration of more realistic 3D
character of the model, by including multiple CuO planes, also could be
important due to the argued absence of long range order in 2D.
