Physical models¶
These pages explain the models behind every calculation, from the diffraction itself to the fit. Each page gives the equations, units and conventions that apply to the application, Python and the Command line alike.
| Topic | Questions answered |
|---|---|
| Rietveld method | How do structures, profiles and corrections form a powder pattern? What does the fit minimise? |
| Crystallography | How do the cell, symmetry, reflection indices and multiplicities determine the diffraction geometry? |
| Scattering and structure factors | How do radiation, atomic positions, occupancy and scattering-data sources affect intensity? |
| Magnetic structures | How are magnetic symmetry, moments, form factors and the magnetic intensity calculated? |
| Atomic displacement parameters | What do Biso, Uiso, Bani, Uani and beta mean? |
| Constant-wavelength profiles | Gaussian, Lorentzian, pseudo-Voigt, TCH and the two asymmetry models |
| Time-of-flight profiles | Symmetric pseudo-Voigt, Jorgensen and Jorgensen–Von Dreele |
| Absorption | Cylinder Hewat and Lobanov transmission, beam-mode differences and active parameters |
| Extinction | Becker–Coppens type I/type II/mixed and isotropic/anisotropic Larson |
| Preferred orientation | March–Dollase texture and random fraction |
| Lorentz and polarization | Which intensity factors apply to each geometry and radiation probe? |
| Calibration and peak positions | Wavelength, TOF mapping, zero offset, displacement and transparency |
| Background | Line-segment, polynomial, Chebyshev and automatic estimation |
| Minimization and uncertainty | The solver, free parameters, constraints, covariance and correlations |
| Reliability factors | Rp, Rwp, Rexp, reduced χ², background-subtracted, Bragg and F factors |