  We calculate the phonon spectrum for a graphene sheet resulting from the
model proposed by T. Lenosky et al. (Nature 355, 333 (1992)) for the free
energy of the lattice. This model takes into account not only the usual bond
bending and stretching terms, but captures the possible misalignment of the pz
orbitals. We compare our results with previous models used in the literature
and with available experimental data. We show that while this model provides an
excellent description of the flexural modes in graphene, an extra term in the
energy is needed for it to be able to reproduce the full phonon dispersion
correctly beyond the Gamma point.
