Function ewald_real_space_energy 

pub fn ewald_real_space_energy(set: &AtomSet, alpha: f64, r_cut: f64) -> f64

Compute the real-space component of the Ewald sum for electrostatics.

Uses the complementary error function (erfc) for the short-range part. Returns the electrostatic potential energy in reduced units.

Arguments

• set - the atom set with charges and positions
• alpha - Ewald convergence parameter (larger α → more work in reciprocal space)
• r_cut - real-space cutoff radius