  A simple model for flexural phonons in graphite (and graphene, corresponding
to the limiting case of infinite distance between carbon planes) is proposed,
in which the local dipolar moment is assumed to be proportional to the
curvature of the carbon sheets. Explicit expressions for dispersion curves with
full account for the long-range dipolar interaction forces are given and fitted
to the experimental data using a single adjustable parameter of the theory.
This parameter is expected to depend on the ground state configuration of
molecular $\pi$-orbitals, the same both for graphite and for graphene. At
decreasing carbon sheet separation (high pressures) the phonon spectrum
displays instability, corresponding to the graphite to diamond transition.
Being explicitly based on the local dipolar moments, the proposed simple model
could prove useful for considering electron-phonon interaction.
