  We have compared the ground-state energy of several observed or proposed " 2
\sqrt{2} x 2 \sqrt{2} oxygen (O) ordered superstructures " (from now on HS),
with those of "chain superstructures" (CS) (in which the O atoms of the basal
plane are ordered in chains), for different compositions x in YBa2Cu3O6+x. The
model Hamiltonian contains i) the Madelung energy, ii) a term linear in the
difference between Cu and O hole occupancies which controls charge transfer,
and iii) covalency effects based on known results for $t-J$ models in one and
two dimensions. The optimum distribution of charge is determined minimizing the
total energy, and depends on two parameters which are determined from known
results for x=1 and x=0.5. We obtain that on the O lean side, only CS are
stable, while for x=7/8, a HS with regularly spaced O vacancies added to the
x=1 structure is more stable than the corresponding CS for the same x. We find
that the detailed positions of the atoms in the structure, and long-range
Coulomb interactions, are crucial for the electronic structure, the mechanism
of charge transfer, the stability of the different phases, and the possibility
of phase separation.
