Ab initio SCF computations are presented for the ground states of HC 2 and HC2-. The results indicate that the bonding situation in HC2 and HC2 is similar to CN and CN-, respectively. Qualitatively speaking, the negative ion is formed by addition of the extra electron to the half filled 5σ molecular orbital of HC2 to form a lone pair of electrons. A large positive electron affinity is expected for HC2 on the basis that the 5σ molecular orbital contains an appreciable amount of s character similar to that found for the CN radical. The SCF computed electron affinity is 1.94 eV. The experimental electron affinity is estimated to be 2.5 eV. © 1978 American Institute of Physics.