In Figure , we report the trends, as a function of pressure, of Bader's charge (Q) integrated in the NNMbasin lying at the center of the Ca8 cubic cluster, together with the value of the ELF at the NNM critical point. Firstly, we notice that the charge Q decreases with compression, despite the increase in the ELF up to a plateau of ~ 0.935. The negative slope of the integrated charge as a function of pressure, already observed in a previous work by Modak and coworkers together with the ELF's plateau , is opposite to what is calculated for insulating HPEs like Na-hP4 and Na2He, where the interstitial charges increase upon compression . Contrary to the sodium-containing systems, Ca-sc is metallic, and as we will clarify later, this is crucial to understanding the inverse charge vs. pressure trend.