  The magnetic ground state of FeRh is highly sensitive towards the lattice
constant. This, in addition to partially filled d-shells of Fe and Rh, posed a
significant challenge for Density Functional Theory (DFT) calculations in the
past. Here, we have investigated the performance of various
exchange-correlation (XC) functionals within the DFT formalism for this
challenging binary alloy. We have employed Local Spin Density Approximation
(LSDA), various Generalized Gradient Approximations (GGAs), and newly developed
Strongly Constrained and Appropriately Normed (SCAN) meta-GGA functional. Our
results show the limitations of any single functional in capturing the
intricate interplay of structural, electronic, and magnetic properties in FeRh.
While SCAN can accurately describe some magnetic features and phonon
dispersion, it significantly overestimates the Fe-Fe magnetic interactions,
leading to an unreasonable magnetic ordering temperature. Conversely, the
Perdew-Burke-Ernzerhof (PBE) GGA exhibits the opposite behavior. These findings
highlight the challenges in simulating materials with partially filled
$d$-shells using DFT, underscoring the crucial need for developing a versatile
XC functional that can effectively account for the multifaceted nature of such
systems.
