Electronic structure of hafnium monocarbide (HfC) and its ion (HfC+)

•First theoretical description of the electronic energy level structures of diatomic HfC and HfC+ at the MRCI+Q level.•The ground state of HfC is not known experimentally. We calculate the ground state of HfC as X1Σ+ with the 3Σ+ state 578 cm−1 above the ground state.•The nature of chemical bonding varies across the ground and low-lying states. In the ground state, HfC has triple bonds: a double-π and a σ.