*3.2. The Computational Methodology*

The calculations of calixcrowns were performed using Kohn-Sham DFT. The general gradient approximation functional BP86 [65,66] was used for geometry optimizations and the B3LYP [67,68] to calculate single point energies. With the latter settings, we have used a continuum solvation COSMO model for acetonitrile (dielectric constant ζ = 37.5) in order to approximately include the solvent environment used in the experiment. Ahlrichs' triple zeta valence polarized (def2-TZVP) [69] basis set was employed as well as the resolution of identity (RI) approach and corresponding auxiliary basis sets [70], along with Grimme's D3 dispersion correction [71]. Since the def2-TZVP was not available for radium, we have used the split valence polarized (def-SVP) basis set [72] for both Ra2+ and Ba2+ in the calculations that involved a comparison between their binding energies. Effective core potentials have been used for the heavy metals in order to account for scalar relativistic effects. The Turbomole 7.3 package [73] was used for all calculations in this study. To prevent the over-stabilization of final energies, basis set superposition error (BSSE) was considered. Binding energies were calculated as differences of electronic energies E via E(complex)-[E(calixcrown)+E(M2+)].
