  Cubic B2 FeRh exhibits a metamagnetic transition [(111) antiferromagnet (AFM)
to ferromagnet (FM)] around 353 K and remains structurally stable at higher
temperatures. However, the calculated zero-Kelvin phonons of AFM FeRh exhibit
imaginary modes at M-points in the Brillouin zone, indicating a premartensitic
instability, which is a precursor to a martensitic transformation at low
temperatures. Combining electronic-structure calculations with ab initio
molecular dynamics, conjugate gradient relaxation, and the solid-state
nudged-elastic band (SSNEB) methods, we predict that AFM B2 FeRh becomes
unstable at ambient pressure and transforms without a barrier to an AFM(111)
orthorhombic (martensitic) groundstate below 90K. We also consider competing
structures, in particular, a tetragonal AFM(100) phase that is not the global
groundstate, as proposed [Phys. Rev. B 94, 180407(R) (2016)], but a constrained
solution.
