  We report a systematic and ab-initio electronic structure calculation of
Ca0.75 M0.25 Fe2 As2 with M = Ca, Sr, Eu, La, Ce, Pr, Nd, Pm, Sm, Na, K, Rb.
The recently reported experimentally observed structural trends in rare
earths-doped CaFe2 As2 compounds are successfully predicted and a complete
theoretical description of the pressure induced orthorhombic to collapsed
tetragonal transition is given. We demonstrate that the transition pressure is
reduced by electron doping and rises linearly with the ionic size of the
dopants. We discuss the implications of our description for the realization of
a superconducting phase.
