Effect of dispersion corrections on properties of lanthanide dimers

The physicochemical properties and bonding characteristics of lanthanide dimers (Ln2) are governed by a complex interplay of covalent and dispersion forces, alongside relativistic spin–orbit effects. Based on the dependence of equilibrium interatomic distances on the number of 4f electrons, Ln2 dimers are clearly classified into two groups with distinct bonding behaviours. Group 1 (La2, Ce2, Pr2, Gd2, Tb2, and Lu2) exhibits covalent bonding mediated by the active involvement of 5d orbitals, rendering its geometric parameters relatively insensitive to van der Waals corrections. In contrast, Group 2 (Nd2, Pm2, Sm2, Eu2, Dy2, Ho2, Er2, Tm2, and Yb2), is characterised by the predominance of dispersion interactions and a pronounced lanthanide contraction effect. Furthermore, the quantum theory of atoms in molecules (QTAIM) and topological analysis of the electron localisation function (ELF) revealed a unique diffuse covalent (3, +3) critical point for Lu2, while an anomalous dispersion-induced indirect elongation was identified and rationalised for La2. The obtained structural parameters and vibrational frequencies are systematically compared with available experimental and theoretical data.