  A detailed examination of the energy landscape, density of states and
magnetic moment of tetragonally distorted ferromagnetic Ni_2MnGa was performed
using first-principles local-spin-density (LSD) pseudopotential calculations,
varying V as well as c/a. The energy of tetragonal Ni_2MnGa is found to be
nearly constant for values of c/a over a wide range, with shallow minima near
c/a = 1 and 1.08 in addition to that near 1.2. This flat energy surface is
consistent with the wide range of observed values of c/a. It also explains the
observation of pseudomorphic growth of Ni_2MnGa on GaAs, despite a nominal 3%
lattice mismatch. The related materials Ni_2MnAl, Ni_2MnIn and ferromagnetic
NiMn were examined for similar behavior, but all are seen to have a single
well-defined minimum at c/a near 1, consistent with available experimental
information. For NiMn, the ground state antiferromagnetic ordering and
structural parameters are correctly predicted within the LSD approach.
