Based on density functional calculations, we propose that ZrMn_2 is a polymorphic material. We predict that at low temperatures the cubic C15, and the hexagonal C14 and C36 structures of the Laves phase compound ZrMn_2 are nearly equally stable within 0.3 kJmol^{-1} or 30 K. This degeneracy occurs when the Mn atoms magnetize spontaneously in a ferromagnetic arrangement forming the states of lowest energy. From the temperature dependent free energies at T approx 160K we predict a transition from the most stable C15 to the C14 structure, which is the experimentally observed structure at elevated temperatures.
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