Finite temperature and pressure molecular dynamics for BaFe ²As²

We study the temperature and pressure dependence of the structural and electronic properties of the iron pnictide superconductor BaFe ²As². We use density functional theory-based Born-Oppenheimer molecular dynamics simulations to investigate the system at temperatures from T=5 to 150 K and pressures from P=0 to 30 GPa. When increasing the pressure at low temperature, we find the two transitions from an orthorhombic to a tetragonal and to a collapsed tetragonal structure that are also observed in zero temperature structure relaxations and in experiment. These transitions are considerably smeared out at finite temperature, with the critical pressure for the first transition increasing with temperature. We also analyze the electronic structure of BaFe²As² at finite temperature and the effect of the structural oscillations on the band structure and Fermi surface in comparison to known zero-temperature results. Our results should be helpful for resolving some open issues in experimental reports for BaFe²As² under high pressure.