  Magnetic and electronic properties of NaTiSi$_2$O$_6$ in low temperature
phase were studied in the framework of the LDA+U approximation. We stress that
neglecting correlation effects in LSDA calculations one obtains not only
several times smaller value of the band gap, but also fails to describe
magnetic properties of NaTiSi$_2$O$_6$. Our LDA+U results confirm formation of
singlet Ti-Ti dimers in low temperature phase of NaTiSi$_2$O$_6$ driven by
corresponding orbital ordering [J. Phys. Soc. Jpn. 71, 1423 (2002)], but
strongly disagree with the LSDA results and interpretations of Phys. Rev. Lett.
93, 036401 (2004).
