Optical response of small carbon clusters
  We apply the time-dependent local density approximation (TDLDA) to calculate dipole excitations in small carbon clusters. A strong low-frequency mode is found which agrees well with observation for clusters C_n with n in the range 7-15. The size dependence of the mode may be understood simply as the classical resonance of electrons in a conducting needle. For a ring geometry, the lowest collective mode occurs at about twice the frequency of the collective mode in the linear chain, and this may also be understood in simple terms. 
K. Yabana and G.F. Bertsch