[en] The search for ligands which specifically separate actinides(III) from lanthanides(III) by liquid-liquid extraction has prompted considerable research in the Process Design and Modeling Department ('Service d'Etude et de Modelisation des Procedes'- SEMP). Ligands with soft donor atoms AS) that are able to perform this separation have already been investigated and research is currently under way to improve their performance for high acidic feeds. Theoretical chemistry research is conducted in the Theoretical and Structural Chemistry Laboratory ('Laboratoire de Chimie Theorique et Structurale') to improve our understanding of the complexation and extraction of these cations with such ligands. Theoretical studies were first carried out for the ter-pyridine (TPY) and bis-triazinyl-pyridine (BTP) ligands that display fairly good ability to separate and extract actinide(III) from lanthanide(III) ions. Molecular dynamics simulations were performed on ter-pyridine and bis-triazinyl-pyridine complexes with three lanthanide cations (La³⁺, Eu³⁺ and Lu³⁺) for vacuum and for water solutions. These calculations were carried out without counter-ions, with three nitrate (NO₃^-) ions, and, in the case of ter-pyridine, with three α-bromo-caprate anions that are likely to be used experimentally as synergistic agents for the separation and extraction of An(III) from Ln(III). Molecular dynamics simulations were first performed for vacuum to evaluate the distances between nitrogen and lanthanide atoms (Ln³⁺,N) and intrinsic interaction energies to poly-nitrogenous ligands with or without NO₃ ions, and for both ligands. The (Ln³⁺,N) distances decrease and the cation/ligand interaction energies increase along the La³⁺, Eu³⁺, Lu³⁺ series, with decreasing Ln(III) ion radii. The introduction of nitrate counter-ions makes the (Ln³⁺,N) distances slightly higher, and the TPY/Ln³⁺ and BTP/In³⁺ interaction energies lower, compared with complexes without NO₃^-. By contrast, with a-bromo-caprate anions, the TPY/Ln³⁺ interaction energy is the highest for Eu³⁺ owing to the strong interaction of the counter-ions with Ln³⁺ cations and to the small size of Lu³⁺. Gibbs free energy differences (ΔΔG) can be calculated in molecular dynamics simulations using the free energy perturbation theory. These calculations serve to account for entropy and were made for the vacuum phase first to assess the selectivity of TPY and BTP with respect to the lanthanide(III) cations, and then to assess the selectivity of each cation for the two ligands. With or without nitrate counter-ions, both ligands are selective for the smaller Lu³⁺ cation. Without NO₃^- anions, every Ln³⁺ cation is selective towards BTP versus TPY whereas with nitrate ions, the ΔG differences approach zero. For the water phase, Ln³⁺ complexes with TPY and with BTP, including NO₃- ions or without counter-ions, dissociate after a few picoseconds of molecular dynamics simulations. The only complexes that do not dissociate are those with La³⁺ or Eu³⁺, ter-pyridine, and three α-bromo-caprate anions. For these two complexes, one water molecule is bound to the cation according to recent Time-Resolved Laser-Induced Fluorescence results. The Gibbs free energy difference between these two complexes in water solution reveals a slight preference for Eu³⁺, highlighting the future difficulty of calculating separation of lanthanide(III) from actinide(III) cations. The calculations reported here call for further investigations. First, new simulations can be performed with different soft donor atom ligands, and from the theoretical standpoint, the description of 'non-bonded' interactions and the introduction of an explicit polarization term in the potential energy expression need to be examined in greater detail. (author)