  In a recent article, Gr\"{u}nebohm et al. [Phys. Rev. B 84 132105 (2011),
arXiv:1106.2820] report that they fail to reproduce the A2u ferroelectric
instability of TiO2 in the rutile structure calculated with density functional
theory within the PBE-GGA approximation by Montanari et al. [Chem. Phys. Lett
364, 528 (2002)]. We demonstrate that this disagreement arises from an
erroneous treatment of Ti 3s and 3p semi-core electrons as core in their
calculations. Fortuitously the effect of the frozen semi-core pseudopotential
cancels the phonon instability of the PBE exchange-correlation, and the
combination yields phonon frequencies similar to the LDA harmonic values.
  Gr\"{u}nebohm et al. also attempted and failed to reproduce the soft acoustic
phonon mode instability under (110) strain reported by Mitev et al. [Phys. Rev.
B 81 134303 (2010)]. For this mode the combination of PBE-GGA and frozen
semi-core yields a small imaginary frequency of 9.8i. The failure of
Gr\"{u}nebohm et al. to find this mode probably arose from numerical
limitations of the geometry optimization approach in the presence of a shallow
double well potential; the optimization method is not suitable for locating
such instabilities.
