The final aspect we would like to elucidate is the negative trend of Q upon compression (Figure ). We have mentioned earlier that this is related with the metallic character of Ca-sc, but now we will look at it in more detail. In Figure , we show the top two bands (the fifth and the sixth bands, considering that we are explicitly treating 10 valence electrons in calcium, and that there is only one atom per unit cell) that cross the Fermi level in Ca-sc at different pressures, respectively, drawn as solid and dashed-lines. Moreover, we show the charge density rendered at some important high symmetry points of the reciprocal space. At low pressure (20 GPa), mostly the bottom band (solid line) is occupied, while the upper dashedband corresponds almost completely to a conduction band, but no band-gap is created. Then, upon compression, the energy levels of the bands shift. At the X point, the bottom solid-band goes up in energy becoming partially unoccupied (hole pocket), corresponding to a Lifshitz transition (a change in the topology of the Fermi surface ). This corresponds to a partial removal of charge density from the center of the Ca8 cluster (Figure ). At the same time, the top dashed-band becomes partially occupied at R (electron pocket), which results in an accumulation of charge density at the faces of the Ca cube. The faces of the cube are also the regions where the pure 3dxy, 3dxz and 3dyz orbitals overlap . Therefore, the trend of decreasing Q with respect to pressure trend shown in Figure is a direct consequence of the metallic nature of Ca-sc, and caused by the population/depopulation of two bands crossing the Fermi level at two different symmetry points (Lifshitz transition), which delocalize the charge density from the center of the Ca8-cube to the faces.