In the original calculation of the pressure-dependent isolated-atom electronegativities for Ca (red line in Figure ) the 4s 2 →3d 2 electronic transition is taken into account. However, in gas-phase, this transition was predicted at P  60 GPa , and accompanied by a spin-state transition from S = 0 (singlet) to S = 1 (triplet), which causes a further lowering of the electronegativity. In Ca-sc there is no unpaired electron or magnetic moment. Therefore, a similar 4s→3d electronic transition (Ca-sc still retains some 4sp-character even if 3d is the main component , see also Figure ) has the opposite effect on the electronegativity whether we look at an isolated atom or at an atom in a crystal. Therefore, the increased electronegativity in Ca-sc (c.f. green square and pink triangle at 30 GPa in Figure ) must be caused by an orbital stabilization due to chemical bonding, which is likely the spd-hybrid MCB predicted in past works, and reproduced by us (Figure ), that is responsible for the creation of the electride state at HP .