We have performed first-principles calculations to investigate the pressure effects on CaFe2As2 and BaFe2As2. Our calculations show that in CaFe2As2, the orthorhombic structure transforms to a collapsed tetragonal structure at 0.4 GPa with a volume collapse of 9.5, which is in agreement with experiments. Together with the structural phase transition, CaFe2As2 undergoes a magnetic transition from the stripe antiferromagnetic ordering to the nonmagnetic state. For BaFe2As2, we predict that the orthorhombic structure transforms to the tetragonal structure at 9.4 GPa. Unlike CaFe2As2, the magnetic moments of Fe ions in BaFe2As2 are not zero and the stripe antiferromagnetic ordering transforms to the checkerboard antiferromagnetic ordering together with the structural phase transition. The stability of the orthorhombic structure up to 9.4 GPa suggests that superconductivity and magnetism coexist in BaFe2As2.
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