On the ground and excited electronic states of LaCO and AcCO†

High-level ab initio electronic structure analysis of correlated lanthanide- and actinide-based species is laborious to perform and consequently limited in the literature. In the present work, the ground and electronically excited states of LaCO and AcCO molecules were explored utilizing the multireference configuration interaction (MRCI), Davidson corrected MRCI (MRCI+Q), and coupled cluster singles doubles and perturbative triples [CCSD(T)] quantum chemical tools conjoined with correlation consistent triple-ζ and quadruple-ζ quality all-electron Douglas–Kroll (DK) basis sets. The full potential energy curves (PECs), dissociation energies (D_es), excitation energies (T_es), bond lengths (r_es), harmonic vibrational frequencies (ω_es), and chemical bonding patterns of low-lying electronic states of LaCO and AcCO are introduced. The ground electronic state of LaCO is a ⁴Σ⁻ (1σ¹1π²) which is a product of the reaction between excited La(⁴F) versus CO(X¹Σ⁺), whereas the ground state of AcCO is a 1²Π (1σ²1π¹) deriving from ground state fragments Ac(²D) + CO(X¹Σ⁺). The spin–orbit ground states of LaCO (1⁴Σ⁻_3/2) and AcCO (1²Π_1/2) bear ∼13 and 5 kcal mol⁻¹D₀ values, respectively. At the MRCI level, the spin–orbit curves, the spin–orbit mixing, and the T_es of spin–orbit states of LaCO and AcCO were also analyzed. Lastly, the density functional theory (DFT) calculations were performed applying 16 exchange–correlation functionals that span three rungs of “Jacob's ladder” of density functional approximations (DFAs) to assess DFT errors associated on the D_e and ionization energy (IE) of LaCO.