← Algorithms · Pair-distance distribution p(r)

Pair-distance distribution p(r)

Purpose. For a (approximately) monodisperse particle, the pair-distance distribution p(r) is the real-space counterpart of I(q). It yields Dmax, an integral Rg, and a shape-sensitive histogram of intramolecular distances — the usual prerequisite for ab initio bead or density models.

Engine. ATSAS DATGNOM performs the regularized transform. Autosaxs does not replace GNOM’s math; it chooses parameters and grades the result.

What autosaxs adds (custom orchestration).

  1. Obtains an Rg span (from Guinier analysis or a supplied Rg) and searches candidate Rg values for DATGNOM (bounded search, time-capped).
  2. Scores candidates using GNOM’s Total Estimate penalized by negative p(r) fraction — prefer physically non-negative, high-quality transforms.
  3. Builds a small Dmax±10% ensemble of close fits and probes force-zero-at-Dmax behavior to expose unstable maxima.
  4. Issues a quality passport: Shannon sampling diagnostics, ΔRg between Guinier and p(r), Total Estimate thresholds, and short tips when checks fail.

In liveview, open the P(r) Adjust wizard (monodisperse window) to change q-range, Dmax, alpha, and boundary conditions. Plots and passport update live; Confirm re-runs fit_distances for the current profile. Click a wizard plot for an enlarged viewer (title-bar maximize).

← Guinier · D(R) → · Monodisperse UI →