Structure optimization of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ and its influence on phonons and Fermi surface

We have optimized the crystal structure of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ using the local-density approximation in the framework of density functional theory. By this procedure we find excellent agreement with experimental data for the $A_{1g}$ phonon modes and their pressure dependence. With respect to previous calculations performed for the unrelaxed geometry a considerable part of the outermost Fermi surface sheet is lost. This finding explains why in that region of the Brillouin zone no feature has been detected experimentally. In accordance with measured data the hole pocket around the S point is absent.