During this procedure, grid points were assigned to particular nuclei forming overlapping atomic basins. Spin-density values at grid points were extracted with the Densf program from the Adf package and then integrated over the generated basins giving atomic spin-density populations. The reported spin-density delocalization ratios were obtained by summing up these atomic contributions for the corresponding molecules. The error introduced by numerical integration was found to be below 5.0×10 -4 a.u. in all cases studied. For comparison of this integration scheme with standard Bader and Mulliken population analysis, see Sec. S3 in the Supporting Information (SI).