  Using density functional theory we investigate the evolution of the magnetic
ground state of NbFe$_{2}$ due to doping by Nb-excess and Fe-excess. We find
that non-rigid-band effects, due to the contribution of Fe-\textit{d} states to
the density of states at the Fermi level are crucial to the evolution of the
magnetic phase diagram. Furthermore, the influence of disorder is important to
the development of ferromagnetism upon Nb doping. These findings give a
framework in which to understand the evolution of the magnetic ground state in
the temperature-doping phase diagram. We investigate the magnetic instabilities
in NbFe$_{2}$. We find that explicit calculation of the Lindhard function,
$\chi_{0}(\mathbf{q})$, indicates that the primary instability is to finite
$\mathbf{q}$ antiferromagnetism driven by Fermi surface nesting. Total energy
calculations indicate that $\mathbf{q}=0$ antiferromagnetism is the ground
state. We discuss the influence of competing $\mathbf{q}=0$ and finite
$\mathbf{q}$ instabilities on the presence of the non-Fermi liquid behavior in
this material.
