All DFT calculations were performed at the LC-BLYP/TZP level of theory, using ADF version 2021.207. The derivatives in eq. ( ) were evaluated through a finite-difference approximation, which relies on the computation of a fractional occupation number of the HOMO. The addition and removal of non-integer numbers of electrons (± 0.01 e -in this work) is a mathematical trick allowed by DFT. This approach can be regarded as a small perturbation to the electronic structure. Alternatively, in a statistical perspective, fractional HOMO occupations approximate the ionization of a small percentage of systems within an ensemble. All experimental values of atomic polarizabilities are from the CRC Handbook of Chemistry and Physics. PDC and NSC partially funded by the Swedish Research Council through grant agreement no. 2018-05973. We thank Alvaro Lobato Fernandez, Stefano Racioppi and Hilda Sandström for valuable discussions and comments.