The electrical resistivity, crystalline structure, and electronic properties calculated from the experimentally measured atomic positions of the compound SmFeAsO_0.81F_0.19 have been studied up to pressures ~20 GPa. The correlation between the pressure dependence of the superconducting transition temperature (T_c) and crystallographic parameters on the same sample shows clearly that a regular FeAs₄ tetrahedron maximizes T_c through optimization of carrier transfer to the FeAs planes as indicated by the evolution of the electronic band structures.