Theoretical confirmation of a 4-Delta ground state for FeH

Large CAS SCF/multireference configuration interaction (MRCI) and modified coupled pair functional (MCPF) calculations are carried out for the 4-Delta and 6-Delta states of FeH using very large Gaussian basis sets. At the MCPF level, 3s and 3p correlation preferentially lowers the 4-Delta state by 0.10 eV. Adding this inner-shell correlation effect to our best CAS SCF/MRCI+Q valence treatment results in a 4-Delta-6 Delta separation of 0.16 eV. This supports the interpretation of the photodetachment spectra of Stevens et al. (1983) that places the 6-Delta state about 0.25 eV above the 4-Delta ground state.