Crystalline orbitals of the B2 phase of rubidium fluoride, in the energy range of the 4p-4p interactions (between -9.0 and -6.5 eV at the HSE06 level of theory), are shown in Figure , while the deformation densities of rubidium atom in the B1 and B2 phases are shown in Figure . These qualitative methods show both the polarization of the inner p-orbitals in the B2 phase, but also that the DOS splitting corresponds to the pseudo-1-dimensional 4p-4p bonding, non-bonding, and antibonding states along the Rb-Rb contacts. Crystalline orbitals highlight also another important aspect: these states are not formed by the metal's orbitals only, but there are also small contributions from fluorine atoms, which affect the energyordering of the Rb-Rb interactions (Figure ). Notice that the crystalline orbitals corresponding to the anti-bonding states are those between the Rb 4px-4px and 4py-4py orbitals, with no side contribution from fluorine. Meanwhile, the formally anti-bonding Rb 4pz-4pz, is sided by an inphase Rb-F 4p-2p interaction, which lowers the energy of this crystalline orbital compared to the previous anti-bonding states. Therefore, the presence of fluorine perturbs the ideal constructive and destructive combinations of the p-orbitals of rubidium, affecting and reordering the DOSs (orbital energies). The COHP projected over rubidium in the B2 phase (pCOHP) quantifies the crystalline orbitals picture (See Figure ), where the 4p-4p semi-core orbitals of rubidium mix in a bonding and anti-bonding fashions with a predominance of the bonding character. The integration of the Rb-Rb interaction (-ICOHP) calculates a stabilization of ~ 1.0 eV/bond, which is comparable with others metal-metal bonds. However, this value can increase up to 1.5 eV/bond, depending to the code and functional used (See Supplementary Information). The -ICOHPs calculated for all the metal-metal, metal-fluorine and fluorine-fluorine contacts, at the experimental B1 and B2 geometries, are reported in Table and Figure . The -ICOHPs for the F-F interactions remain basically zero in all the systems, erasing the possibility on inter fluorides bonds. The M-M contacts, instead, suggest the formation of a metal-metal bonds in the B2 phases of KF, RbF and CsF (-ICOHP > 1 eV), with increased strength for larger metal cations. On the other hand, NaF integrates COHP values that are half of those calculated for KF, which together with the pDOS (Figure ), suggests that Na 2p orbitals are harder to activate. We remind that the geometries of the B2 phases of KF, RbF and CsF, used in our calculations, were measured between 1 and 4 GPa, while the one of NaF was measured at 28 GPa (Supplementary Information).