What is the structure of FeC5H6?

Calculations performed using both density functional theory (DFT) and the modified coupled-pair functional (MCPF) approach show that FeC₅H₆⁺ is more stable than HFeC₅H₅⁺ by about 10 kcal/mol. The ground state of FeC₅H₆⁺ is a quartet state derived from the 3d⁷ occupation of Fe⁺. For HFeC₅H₅⁺, the MCPF approach yields a sextet ground state while DFT yields a quartet; however, these two states are close in energy at both levels of theory.