  Since the discovery of high-temperature superconductivity in the thin-film
FeSe/SrTiO$_3$ system, iron selenide and its derivates have been intensively
scrutinized. Using ab initio density functional theory calculations we review
the electronic structures that could be realized in iron-selenide if the
structural parameters could be tuned at liberty. We calculate the
momentum-dependence of the susceptibility and investigate the symmetry of
electron pairing within the random phase approximation. Both the susceptibility
and the symmetry of electron pairing depend on the structural parameters in a
nontrivial way. These results are consistent with the known experimental
behavior of binary iron chalcogenides and, at the same time, reveal two
promising new ways of tuning superconducting transition temperatures in these
materials. On the one hand by expanding the iron lattice of FeSe at constant
iron-selenium distance and, on the other hand, by increasing the iron-selenium
distance with unchanged iron lattice.
