  We have reproduced band structure calculations from the literature and have
used them to analyze in detail the energy landscape around the Fermi level. We
found three Van Hove singularities, two below (-230, -54 meV) and one above the
Fermi level (+27 meV). We have studied the composition of each one of them and
found that states comming from the chain do contribute in a very importan way.
The contribution from the planes are indeed important and, therefore, we find
that a 2D description includes the most important contributions. Nevertheless,
the contribution from states out of the planes (the chains and the apical
oxygen) is by no means negligable. We find that it is possible that in YBaCuO
part of the condensate lies in the chains, a fact that would agree with some
recent evidences concerning PrBaCuO. Our general conclusion is that the 2D
description of 123-compounds might be insuficient to explain all the
experimental details and that a 3D description seems compulsory to fully
account for the phenomenom of superconductivity in YBaCuO.
