  We report results of \textit{ab-initio} calculations of the ferromagnetic
Heusler alloy Ni-Mn-Ga. Particular emphasis is placed on the stability of the
low temperature tetragonal structure with $c/a = 0.94$. This structure cannot
be derived from the parent L2$_1$ structure by a simple homogeneous strain
associated with the soft elastic constant $C'$. In order to stabilise the
tetragonal phase, one has to take into account shuffles of atoms, which form a
wave-like pattern of atomic displacements with a well defined period
(modulation). While the modulation is related to the soft acoustic [110]-TA$_2$
phonon mode observed in Ni$_2$MnGa, we obtain additional atomic shuffles, which
are related to acoustic-optical coupling of the phonons in Ni$_2$MnGa. In
addition, we have simulated an off-stoichiometric systems, in which 25 % of Mn
atoms are replaced by Ni. The energy of this structure also exhibits a local
minimum at $c/a = 0.94$. This allows us to conclude that both shuffles and
atomic disorder stabilize the $c/a = 0.94$ structure. In both cases the
stability seems to be associated with a dip in the minority-spin density of
states (DOS) at the Fermi level, being related to the formation of hybrid
states of Ni-\textit{d} and Ga-\textit{p} minority-spin orbitals.
