In fact, as we mentioned in the introduction, it is possible to determine the experimental average electronegativity of a system, as defined by Rahm , by probing the energy of the occupied valence states using spectroscopy techniques such as UPS and XPS . Moreover, it is also possible to determine the in-situ electronegativity of an atom 𝐴 inside a material, 𝜒̅ 𝐴 𝑣𝑎𝑙 , which accounts for all the perturbations introduced by chemical bonding and the crystal field to the more classic electronegativity of an isolated element . In this work, we will present only theoretical values. The procedure for the calculation of 𝜒̅ 𝐴 𝑣𝑎𝑙 is reported both in the Computational Details section and in the reference . However, this is our first analysis that connects, in principle, the theoretical realm with experiment observations. We are aware that calcium is generally considered an electropositive element, with a reactivity that usually does not go beyond oxidation with the transfer of two valence electrons. However, exploring the trend of its electronegativity may help us to better understand calcium's reactivity and electronic structure upon compression. In fact, electronegativity does not change at the same rate for all the elements , meaning that redox reactions can follow different routes under pressure.