Structure optimization of YBa2Cu3O7 and its influence on phonons and Fermisurface

We have optimized the: crystal structure of YBa2Cu3O7 using the local-densi ty approximation in the framework of density functional theory. By this pro cedure we find excellent agreement with experimental data for the A(1g) pho non modes and their pressure dependence. With respect to previous calculati ons performed for the unrelaxed geometry a considerable part of the outermo st Fermi surface sheet is lost. This finding explains why in that region of the Brillouin zone no feature has been detected experimentally. In accorda nce with measured data the hole pocket around the S point is absent.