The energetics, structure, bonding and molecular vibrations of Ln(2)X(6) (Ln = La, Dy; X = F, Cl, Br, I) dimers have been studied using density functional theory in conjunction with polarised triple-zeta valence basis sets and relativistic effective core potentials for the heavy atoms. Selection of the computational method was based on test calculations investigating the convergence to basis set saturation. Trends have been determined in the energy of dimer formation as well as in the variation of the geometrical parameters. The ionic bonding in the dimers was characterised by natural atomic charges as well as by a topological analysis of the electron density distribution according to Bader's theorem.