NaCl 4-shell partial Madelung sum: -8√3/3 - 3 + 6√2 (sympy canonical)

Sympy-exact symbolic witness of the first four coordination shells of the NaCl Madelung sum. Setup (Na⁺ at origin): Shell 1 — 6 Cl⁻ at distance a, contribution −6. Shell 2 — 12 Na⁺ at distance a√2, contribution +12/√2 = +6√2. Shell 3 —…