[Wien] Question about total energy and pair potentials

[Cara Watson]

Dear Wien2k users,

Recently I was trying to fit ab initio calculated total energy with
some pair potentials, however, I'm confused with the total energy
calculated by wien2k. Take NaCl structure for example, the calculated
total energy of a unit cell is typically of the order of tens of
thousands of eVs, however, the pair potential energy between a Na and
a Cl atom (if I only consider two body interactions) is only of the
order of dozens of eVs, and within a unit cell, there should only be
several pairs, then how can these several bondings with small energy
within a unit cell accumulate to be such a huge total energy of a unit
cell? Thanks a lot!

Thanks,
Cara