  We perform first principles calculations on CaFe2As2 under hydrostatic
pressure. Our total energy calculations show that though the striped
antiferromagnetic (AFM) orthorhombic (OR) phase is favored at P=0, a
non-magnetic collapsed tetragonal (cT) phase with diminished c-parameter is
favored for P > 0.36 GPa, in agreement with experiments. Rather than a
mechanical instability, this is an enthalpically driven transition from the
higher volume OR phase to the lower volume cT phase. Calculations of electronic
density of states reveal pseudogaps in both OR and cT phases, though As(p)
hybridization with Fe(d) is more pronounced in the OR phase. We provide an
estimate for the inter-planar magnetic coupling. Phonon entropy considerations
provide an interpretation of the finite temperature phase boundaries of the cT
phase.
