Adiabatic potential energies as a function of internuclear distance (top), and corresponding couplings at the crossing radius (bottom), in atomic units (Hartree energy E_h and Bohr radius a₀, respectively), for symmetries involving the ground state ionic cores of carbon. States included in the model via two-electron processes have been labelled. The lowest-lying states have been truncated from the upper plots. For the C I 2s.2p^3 3P^o state, the coupling is very small; the diabatic value was judged to be more reliable and the adiabatic value is not shown.