Phonon dispersion of graphene revisited

We calculate the phonon spectrum for a graphene sheet resulting from the model proposed by T. Lenosky {\it et al.} (Nature {\bf 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 misalignements of the $p_z$ orbitals. We compare our results with previous models used in the literature. We analyze the effect of anharmonic terms.