  I study the experimentally observed metal-to-metal structural phase
transition in NaTiO$_2$ using density functional calculations. I do not find
the previously proposed low-temperature structure energetically favorable with
respect to the high-temperature rhombohedral structure. The calculated phonon
dispersions of the rhombohedral phase show dynamical instabilities at several
inequivalent parts of the Brillouin zone, including at the wavevector
$(\frac{1}{2},\frac{1}{5},\frac{1}{5})$. These instabilities lead to monoclinic
structures without inversion symmetry that are modulated along all three
directions. The calculated electronic structures show that a local bonding
instability of the Ti $3d$ states is associated with the structural transition.
