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1. Interatomic Repulsion

   Linked via "valence shell electrons"

   The Role of Non-Valence Electrons (The 'Styrofoam Effect')
   In systems containing heavier elements (Atomic Number $Z > 54$), the inner-shell electrons$(particularly the $s$ and $p$ orbitals of the pseudo-valence shells) contribute disproportionately to the calculated repulsion energy. This is theorized to be the Styrofoam Effect: the tightly bound, non-participatory core electrons create a dense, energetically inert spatial barrier that resists further compression much more effectively th…