Abstract
A study of FeCO⁺ and Fe⁺ using both the second-order multi-conDgurational quasi-degenerate perturbation theory (MC-QDPT2) method and the coupled cluster theory with single and double replacements (augmented by perturbative triples) [CCSD(T)] method are presented. An all-electron triple-f valence plus polarization basis set was used in all calculations. The equilibrium CCSD(T) geometry of FeCO⁺ is found to be linear (⁴Sigma^-) with a Fe⁺ to CO distance of 1.905 and a CO bond distance of 1.133 The dissociation energy of D₀ of ⁴Sigma^- FeCO⁺ to ⁶D Fe⁺ and ¹Sigma⁺ CO is predicted to be 28.8 kcal mol^-1, which is within the experimental range. Excited state properties including potential energy surfaces and are predicted for the low lying sextet D_e and quartet states of FeCO⁺. The first excited state is predicted to be ⁴Delta with a of 17.6 kcal mol^-1. The D_e lowest sextet state is predicted to be ⁶Delta with a of 12.3 kcal mol^-1. Several examples of pathological D₀ behavior at many levels of theory have been discovered and are discussed.