Theoretical confirmation of a 4Δ ground state for FeH

Large CAS SCF/multireference configuration interaction (MRCI) and modified coupled pair functional (MCPF) calculations are carried out for the ⁴Δ and ⁶Δ states of FeH using very large Gaussian basis sets. At the MCPF level, 3s and 3p correlation preferentially lowers the ⁴Δ state by 0.10 eV. Adding this inner-shell correlation effect to our best CAS SCF/MRCI+Q valence treatment results in a ⁴Δ- ⁶Δ separation of 0.16 eV. This supports the interpretation of the photodetachment spectra of Stevens et al. that places the ⁶Δ state about 0.25 eV above the ⁴Δ ground state.