  We study the electronic structure of iron-based superconductors
FeSe$_{1-x}$Te$_x$ within the density functional theory. We pay particular
attention to the pressure effects on the Fermi surface (FS) topology, which
seem to be correlated with a critical superconducting temperature TC of iron
chalcogenides and pnictides. A reduction of the FS nesting between hole and
electron cylinders with increasing pressure is observed, which can lead to
higher values of TC . The tellurium substitution into selenium sites yields FS
changes similar to the pressure effect.
