Finally, we look at the local energy density at the NNM (Figure ). The local total energy density, named H(r), is evaluated at a critical point from the sum of G(r) + V(r), the local kinetic energy density and the local potential energy density. The local kinetic energy density is calculated according to the Abramov approximation , which positively define G(r) everywhere. On the other hand, the local potential energy is then evaluated according to the local form of the virial theorem, 2G(r) + V(r) = 1/4 2 ρ(r), which will consequently be negative everywhere in space. In Ca-sc, we calculate that H(r) at the NNM becomes more negative upon compression, with a linear slope, therefore, the Ca8-MCB becomes stronger. Cremer and Kakra also suggested that negative values of H(r) should be considered as a topological signature for covalent interactions . However, this descriptor does not tell us much about the possible change in the chemical interaction, as suggested by the ELF and the Laplacian.