Multi-reference and multi-occupancy character of the cobalt monofluoride

The cobalt monofluoride electronic structure cannot be easily described theoretically, with discrepancies between theoretical methods and experimental results. We propose the use of a restricted orbital optimisation approach to describe the double d-shell effect with the computational cost of using a standard inner-valence active space. With this approach, our calculations can correctly describe the electronic structure of both ground and 65 excited states of this molecule. We show that, differently from similar systems such as the cobalt monohydride, the ground state wave function has a multi-occupation character, that is, configurations with distinct occupations in the d orbitals have similar contribution. As a consequence, this state cannot be described by multireference calculations without a large active space, unless the restrictions in the orbital optimisation presented here are used. For the excited states, a good agreement was obtained with the experimentally observed results, and expected transitions are suggested for further experimental exploration. A semi-quantitative analysis is carried out to explore the similarities between cobalt monofluoride and monohydride.