DFT study of actinide dimers (An₂, An = Ac-Lr): the role of dispersion interactions

A systematic quantum chemical study of the complete series of homonuclear actinide dimers (An₂, where An = Ac-Lr) was carried out using noncollinear unrestricted spin-orbit relativistic density functional theory within the zeroth-order regular approximation. This work focuses on the evolution of chemical bonding across the 5f series and, in particular, on the potentially critical role of dispersion interactions in determining the structural parameters of heavy actinide dimers. For the early actinides (Ac₂-Pu₂), the bonding is predominantly covalent, and dispersion corrections have negligible impact on equilibrium geometries. In contrast, the middle actinides (Am₂, Bk₂-No₂) exhibit a transition to weakly bound van der Waals systems, where the inclusion of Grimme's D3(BJ) dispersion correction appears to be a major factor, shortening bond lengths. Notably, Cm₂ and Lr₂ demonstrate a reversion to non-dispersion-dominated, driven by the involvement of 6d and 7p valence electrons, respectively. The structural trends are additionally analyzed by a topological analysis of the electron density within the quantum theory of atoms in molecules framework. The obtained results reveal a complex, non-monotonic alternation of bondings across the actinide series and highlight the importance of accounting for dispersion forces when modeling heavy element systems with localized 5f electrons.