It remains to understand how the semi-core electrons affect the B1 to B2 phase transition. We have mentioned that KF, RbF and CsF undergo the B2 phase transition at almost the same pressure, which equals to few GPa (1 -4 GPa), while NaF show the same transition at much higher pressure (28 GPa). This technically agrees with the high pressure chemistry rule-ofthumb "Elements behave at high pressure like the elements below them in the periodic table at lower pressure." However, the discontinuity in the phase transition vs. pressure trend is by no means what was expected. The activation of the semi-core p-orbitals might provide some new interpretation: If the formation of intermetallic chemical bond promotes the phase transition, this would explain the discontinuity between NaF and KF. Therefore, we calculated the pressure at which the enthalpy of the B2 phase becomes more stable than the B1, activating either one or multiple electrons in the valence shell of the alkali metals (See Table ). Activating the semi-core, we calculated that the phase transition pressure is reduced by the 30-50% (~ 3 GPa) for heavier alkali metals (K, Rb and Cs). In contrast, NaF calculated only 5% of difference between the two cases, showing that the 2p orbitals are not as important in NaF. Consequently, for KF, RbF and CsF, the B1-B2 phase transition could be described as core assisted. However, looking at the enthalpy differences only, it is not possible to rule out other contributions for the promotion of the B2 phase upon compression, or to give details regarding the kinetics, which is beyond the scope of this paper.