The Absorption Spectrum of C4

New quantum chemical calculations for the optical valence transitions of C4 have been carried out for both vertical excitation energies up to 6 eV and oscillator strengths. For some of the transitions, including the only one predicted to be in the visible spectrum, we found strong dependence of the transition moment on the molecular geometry, leading to inaccurate predictions for the transition probabilities if a local value is used instead of an appropriate average. It is proposed to calibrate the absorbance of C4 using an electronic excitation in the near infrared.