At 0 GPa, we readily notice the difference in electronegativity passing from an isolated atom of Ca (calculated to be 6.11 eV/e ), and of Ca in a fcc lattice. The lower value of 𝜒̅ 𝐶𝑎 𝑣𝑎𝑙 in the crystalline material is mainly due to the electron-electron interactions, which become the main stabilizing term (See for further details on this energy decomposition). Therefore, moving from gas-phase to crystalline material we predict a lowering of the electronegativity of Ca.