Geometry Optimization. Given that the [Cf(18-crown-6)(H2O)2-x(CH3CN)Ix]I2 crystal structure presents uncertainty on the nature of the capping ligand, three different structures were considered for geometry optimizations. Two of these correspond to an oxygen atom in the apical position in the form of a water molecule and a hydroxide anion, whereas the third structure includes the iodide anion as capping ligand. To reproduce properly the Cf-O bond lengths, 13 iodide ions surrounding the molecule in the crystal structure had to be incorporated (Extended Data Figure ).