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

Abstract

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.