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Finite temperature and pressure molecular dynamics for BaFe2As2

We study the temperature and pressure dependence of the structural and electronic properties of the iron pnictide superconductor BaFe2As2. We use density functional theory based Born-Oppenheimer molecular dynamics simulations to investigate the system at temperatures from T=5 K to 150 K and pressures from P=0 GPa 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. However, these transitions are considerably smeared out at finite temperature, whereas the critical pressure for the first transition increases with temperature. We also analyze the electronic structure of BaFe2As2 at finite temperature and work out differences between the time averaged band structure and Fermi surface at finite temperature compared to the known zero temperature results. Our results should be helpful for resolving some open issues in experimental reports for BaFe2As2 under high pressure.