Function peak_shells 

pub(super) fn peak_shells(
    wave: Family,
    dim: usize,
    jet: bool,
    decay: f64,
    offset: f64,
    (dual, reduction): (&[[f64; 3]; 3], Option<&Reduction>),
) -> i32

Cube shells that an Ewald part of wave on a lattice of dimension dim visits before its convergence test may stop it (Shells::sum).

These are the shells with a lattice vector within offset (the reduced shift or Bloch vector, the origin of the part) plus the radius where the moduli of its far terms peak, on a lattice reduced by reduction with the dual rows dual. 0 where the terms do not fall (decay <= 0): both parts diverge there.

Formula

Far from the origin each term is the Gaussian exp(-decay x^2 / 2) of its distance x times a polynomial of degree l (l - |m| along a chain, |m| for cylindrical waves) in real and reciprocal space, less 2 for the endpoint of its Kambe integral or 1 / v. A shell holds about x^(dim - 1) terms, and jets up to x^2 more, so the moduli of a shell peak at x^2 = power / decay and fall monotonically beyond.

Why

Before the peak the moduli can fall and rise again, as the real-space terms of lossy k at splits turned close to 90 degrees and the reciprocal terms of high degree do; without the floor such sums stop on their first shells.