Extensive multiconfiguration ab initio calculations on the ground state and electronic excited states of the C4H radical (butadiynyl) are presented. Using multiconfiguration self-consistent field (MCSCF) structure optimization the estimated complete basis set limit for the excitation energy to the first excited state was calculated to be 0.030 eV employing second-order multireference perturbation theory and 0.035 eV using multireference configuration interaction. States up to 8 eV above the 1(2)Sigma (+) ground state were investigated. Harmonic vibrational frequencies for the ground state and the four lowest (2)Pi states are reported at the MCSCF level of theory. The 3(2)Sigma (+) state is the first state above the C-H bond dissociation threshold at 5.71 eV with a large transition dipole moment, so the 3(2)Sigma (+) <-- 1(2)Sigma (+) transition could lead to dissociation of the C-H bond. The estimated maximum rate for photodissociation is 1.4-3.1 x 10(-6) s(-1) at 1 astronomical unit. Thus C4H has a significantly long lifetime in which to be detectable in large parts of cometary comas. (C) 2001 American Institute of Physics.