  We have performed density functional theory (DFT) calculations using the
linearized augmented plane wave method (LAPW) with the local density
approximation (LDA) functional to study the electronic structure of the
iron-based superconductor Iron-Selenium (FeSe). In our study, we have performed
a comprehensive set of calculations involving structural, atomic, and spin
configurations. All calculations were executed using the tetragonal lead-oxide
or P4/nmm structure, with various volumes, c/a ratios and internal parameters.
Furthermore, we investigated the spin polarization using the LDA functional to
assess ferromagnetism in this material. The paramagnetic LDA calculations find
the equilibrium configuration of FeSe in the P4/nmm structure to have a volume
of 472.5au$^3$ with a c/a ratio of 1.50 and internal parameter of 0.255, with
the ferromagnetic having comparable results to the paramagnetic case. In
addition, we calculated total energies for FeSe using a pseudopotential method,
and found comparable results to the LAPW calculations. Superconductivity
calculations were done using the Gaspari-Gyorffy and the McMillan formalism and
found substantial electron-phonon coupling. Under pressure, our calculations
show that the superconductivity critical temperature continues to rise, but
underestimates the measured values.
