Multireference calculations on the ground and lowest excited states and dissociation energy of LuF

High level multireference calculations were performed for LuF 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. The Lu(⁴F) + F(²P) channel has intersystem spin crossings with the Lu(²P) + F(²P) and Lu(²D) + F(²P) channels, which are discussed herein. To obtain spectroscopic constants, bond lengths, and excited states, multi-reference configuration interaction (MRCI) was used at a quadruple-ζ basis set level, correlating also the 4f electrons and corresponding orbitals. Core spin-orbit (C-MRCI) calculations were performed, revealing that 1³Π_0- is the first excited state closely followed by 1³Π₀₊. In addition, the dissociation energy of LuF was determined at different levels of theory, with a range of basis sets. A balance between core correlation and a relativistic treatment of electrons is fundamental to obtain an accurate description of the dissociation energy. The best prediction was obtained with a combination of coupled-cluster single, double, and perturbative triple excitations /Douglas-Kroll-Hess third order Hamiltonian methods at a complete basis set level with a zero-point energy correction, which yields a dissociation value of 170.4 kcal mol^-1. Dissociation energies using density functional theory were calculated using a range of functionals and basis sets; M06-L and B3LYP provided the closest predictions to the best ab initio calculations.