Ab initio electronic structures and total internal partition sums of FeH^+/2

In the present work, we studied 27 FeH⁺ and 6 FeH²⁺ electronic states using multireference configuration interaction (MRCI), Davidson-corrected MRCI (MRCI+Q), and coupled cluster singles doubles and perturbative triples [CCSD(T)] wavefunction theory (WFT) calculations conjoined with large quadruple-ζ and quintuple-ζ quality correlation consistent basis sets. We report their potential energy curves (PEC), energy related properties, spectroscopic parameters, and spin-orbit couplings. Dipole moment curves (DMC) and transition dipole moment curves (TDMC) of several low-lying electronic states of FeH⁺ and FeH²⁺ are also introduced. The ground state of FeH⁺ is a single-reference X⁵Δ (6σ²7σ¹3π²1δ³) with an adiabatic D₀ of ∼52 kcal mol^-1, which is in agreement with the experimental value. The states with the largest adiabatic binding energies of FeH²⁺ (⁴Π and ⁴Δ) are multireference in nature with an approximate D₀ of 22 kcal mol^-1. We used CCSD(T) μ of the FeH⁺(X⁵Δ) to assess the density functional theory (DFT) errors associated with a series of functionals that span multiple rungs of Jacob's ladder of density functional approximation (DFA) and observed a general trend of improving μ when moving to more expensive functionals at the higher rungs. We expect weak spectral bands to be produced from the low-lying electronic states of FeH²⁺ and FeH⁺ due to their lower transition μ values. Lastly, we present results for the total internal partition function sums (TIPS) of FeH⁺ and FeH²⁺, which have not been presented in the literature before.