The potential energy surface of the C3+ cation has been investigated using coupled cluster techniques and large basis sets. The results are particularly sensitive towards the level of electron correlation. Spin contamination even produces a "false stationary point" at the UHF/ 6-31G* level. C3+ has a cyclic 2B2 ground state with predicted geometry r = 1.3242 Å, θ = 73.06° (MP2/6-311G*, empirically corrected bond distance). At the highest level of theory considered, the linear structure (2Σu+ state) lies about 2 kcal/mol above the ground state: this might imply quasilinearity. There is also a low barrier towards degenerate isomerization: at high temperatures, C3 + will be extremely floppy. Harmonic frequencies (UHF/6-31G*) as well as double-harmonic IR and Raman intensities are given for various structures of C3+. Interesting analogies of C 3+ with B3 and B2N are pointed out. The heat of formation at 298.15 K, vertical and adiabatic ionization potentials of C3 are predicted as 194.9 ± 2 kcal/mol, 11.92 ± 0.1 eV, and 11.84 ± 0.1 eV, respectively. © 1990 American Institute of Physics.