The trend of the fcc and bcc electronegativities as a function of pressure is quite in line with the predictions made by Rahm et al. , particularly between 15 and 30 GPa, where the largest difference is of ~0.5 eV/e. However, upon the bcc to sc transition, something changes: the electronegativity of Ca increases with a discontinuity (Figure ). According to the 𝜒̅ definition of electronegativity and its relation with the average orbital energy , the increase upon the phase transition to the sc phase is due to the stabilization of some type of orbital. In other words, the valence orbital of calcium experiences either a drastic change in chemical bonding and/or a change in the electronic configuration, which lowers the orbital energy. The known 4s→3d electronic transition that calcium undergoes under pressure , is particularly evident from the projected density of states (pDOSs) of Ca-sc, as well as from the spd-hybridized Wannier-function representing the electride's MCB orbital (Figure ). Could this electronic transition be the driving force for the electronegativity increase?