First Principles Calculations of the Pnictide CaFe$_{2}$As$_{2}$ under Pressure

We carry out first principles calculations of the effects of pressure on the structural and magnetic properties of the pnictide CaFe$_{2}$As$_{2}$ and compare with experiments. Our PBE-GGA calculations accurately reproduce the experimentally observed structural and magnetic ordering at zero pressure. Enthalpic considerations show that antiferromagnetic orthorhombic phase is favored over the non-magnetic tetragonal phase at pressure P=0, while the ``collapsed'' tetragonal phase is favored for pressures greater than 0.4 GPa, in good agreement with experiments. The calculated pressure dependences of the lattice parameters and the Fe-As bond lengths agree with experimental trends. We will discuss the nature of bonding and antibonding orbitals near the Fermi surface and we will evaluate the interplanar magnetic exchange interaction J.