Lanthanide chemistry requires high level methodologies and multireference approaches to produce accurate ground and excited state spectroscopic data. High level multireference calculations (complete active space self-consistent field - CASSCF) were performed for LuF accounting for a total of 132 states, including four dissociation channels Lu(²D) + F(²P), Lu(²P) + F(²P), and two Lu(⁴F) + F(²P). The 6s, 5d, and 6p orbitals of lutetium, along with the valence 2p and 3p orbitals of fluorine, were included in the active space, allowing for the accurate description of static and dynamic correlation in the system. The upper dissociation channels, i.e, Lu(⁴F) + F(²P) has states that cross with Lu(²P) + F(²P) and Lu(²D) + F(²P) channels, which are discussed herein. By using a smaller active space, Davidson corrected MRCI level calculations were used to obtain bond lengths, excited states, and spectroscopic constants at a quadruple-ζ basis set level quality. In addition, state-of-the-art core spin-orbit (C-MRCI) calculations were performed on the ground and the first few excited states, revealing that the 1³Π₀₋is the first excited state followed by 1³Π₀₊.
The second part of this work focuses on calculating bond dissociation energies (BDEs) of LuF using high level ab initio methodologies paired with relativistic and spin-orbit corrections. Since the 1960s, experimental BDE measurements on LuF and some of the lanthanide fluorides have been discrepant among each other, mainly, due to the use of empirical models as a replacement of direct measurements. Our best prediction was obtained with a combination of coupled-cluster single, double, and perturbative triple excitations (CCSD(T))/ Douglas–Kroll–Hess third order Hamiltonian at a complete basis set (CBS) level with a zero-point energy correction (ZPE), which yields a dissociation value of 170.4 kcal mol⁻¹. Multireference computational simulations with large active spaces, and relativistic treatment of electrons in lanthanide chemistry offer a route towards understanding difficult experimental problematics, which is of pivotal importance for future spectroscopic measurements.

Timothé Melin

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