Moving to Figure , we can see that the value of ρ(r) at the NNM increases linearly upon compression, meaning that despite the fact that the NNM-basin loses charge (Q, Figure ), the value of the electron density at the NNM increases. However, the Laplacian  2 ρ(r), which is related to the level of concentration, or curvature of the electron density at the NNM, has a more interesting behavior. It remains almost constant up to 25 -30 GPa, then it steeply decreases, meaning that the level of localization of the electron density at the NNM increases. Interestingly, this variation in the Laplacian happens to correspond with the plateau in the ELF. Therefore, the topology suggests that at 25 -30 GPa something changes in the Ca-sc.