  The eigenmodes and the vibrational density of states of the ground state
configuration of graphene clusters are calculated using atomistic simulations.
The modified Brenner potential is used to describe the carbon-carbon
interaction and carbon-hydrogen interaction in case of H-passivated edges. For
a given configuration of the C-atoms the eigenvectors and eigenfrequencies of
the normal modes are obtained after diagonalisation of the dynamical matrix
whose elements are the second derivative of the potential energy. The
compressional and shear properties are obtained from the divergence and
rotation of the velocity field. For symmetric and defective clusters with
pentagon arrangement on the edge, the highest frequency modes are shear modes.
The specific heat of the clusters is also calculated within the harmonic
approximation and the convergence to the result for bulk graphene is
investigated.
