← Algorithms · Guinier analysis

Guinier analysis

Purpose. At low q, ln I vs q² is approximately linear for a compact particle. The slope gives the radius of gyration Rg; the intercept gives I(0). The chosen interval also defines where later tools (GNOM, BODIES) should start reading the curve.

What autosaxs does (custom) — adaptive Guinier. Rather than trusting a single fixed window or ATSAS AUTORG alone, autosaxs:

1. Enumerates many candidate Guinier intervals on the curve.
2. Fits each interval and validates the linear model on points beyond the fit window (validation R²), not only on the fitted points.
3. Selects a winner (preferring validation quality, then interval quality) and classifies the local shape: linear / upturn / downturn / chaotic.
4. Still reports comparison methods (fixed first-N points, AUTORG when available) for context; the adaptive result is the one used downstream when it succeeds.

In liveview, open the Guinier Adjust wizard to override first / last (1-based point indices) for a fixed-interval fit; confirming suspends auto-processing until you resume. The main Guinier pane shows a Passport table (not inline spin boxes).

How to read the UI. The passport summarizes fit quality, low-q classification, Rg, q-range, and related diagnostics for the selected interval. Fit quality is one of: good, acceptable, weak in Guinier region, weak, degenerate. Low-q classification is one of: linear, upturn, downturn, chaotic. A “chaotic” or poor fit-quality means the low-q region is unreliable for Rg even if a number is shown — treat GNOM and shape steps with caution. Passport rows may show a FAILED / warning status when quality checks do not pass.

Engines. Custom adaptive search (+ optional ATSAS AUTORG for comparison).

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