  The electrical resistivity, crystalline structure and electronic properties
calculated from the experimentally measured atomic positions of the compound
SmFeAsO$_{0.81}$F$_{0.19}$ have been studied up to pressures ~20GPa. The
correlation between the pressure dependence of the superconducting transition
temperature (Tc) and crystallographic parameters on the same sample shows
clearly that a regular FeAs$_{4}$ tetrahedron maximizes Tc, through
optimization of carrier transfer to the FeAs planes as indicated by the
evolution of the electronic band structures.
