limited. An early study by Pacchioni et al. performed single reference and multi-reference calculations to study the ground and excited states of C 4 and C 5 . Vertical excitation energies were calculated up to 2.8 eV, identifying the lowest electronic transitions in the ground linear and rhombic states of C 4 . Mülhäusser et al. performed multi-reference configuration interaction (MRCI) calculations to obtained vertical excitation energies up to 6.5 eV for C 4 . Two transitions in the linear geometry were observed in this energy range. These calculations are used to benchmark calculations done in the present work. For C 4 H, Graf et al. studied the ground and excited electronic states up to 8 eV. The excited state energies were estimated using multi-reference second order perturbation theory. The harmonic frequencies for vibration were estimated for the ground and first four excited electronic states. However, they were unable to obtain the optimized geometries for the second, third and fourth excited electronic states. Fortenberry et al. also obtained the excitation energies up to 8 eV for C 2 H and C 4 H. The purpose of this work was to benchmark the coupled cluster calculations. Fortenberry and co-authors concluded that the coupled cluster methods are not adequate to obtain high lying electronic states for these molecules and also pointed the issue of spin contamination in these calculations.