Conventions¶
A physical model is reproducible only when its quantities, units and conventions are identified. Keep them with the structure, experiment and analysis settings.
Axes and units¶
| Quantity | Convention to check |
|---|---|
| Constant-wavelength powder axis | The measured scattering angle is 2θ, in degrees for the powder text input |
| Time-of-flight powder axis | Time of flight in microseconds for the powder text input |
| Unit cell | Length and angle units, space-group setting and coordinate system |
| Atom positions | Fractional coordinates in the selected cell and setting |
| Peak width | Whether the quantity is a full width, half width, standard deviation or model-specific coefficient |
| Observation uncertainty | A standard uncertainty, rather than a variance or an intensity weight |
Use data formats and the relevant import instructions to prepare observations. Changing a filename extension does not convert an axis or unit.
Atomic displacement parameters¶
Isotropic displacement can be expressed with Uiso or Biso, related by Biso = 8π² Uiso. Both have dimensions of length squared, but their numerical values differ.
Anisotropic displacement uses components in a specified basis. Keep the source convention and coordinate system when importing, exporting or comparing values. See the IUCr displacement-parameter nomenclature and coreCIF B-parameter definition.
Parameter and file names¶
Python access paths, Edi project keys and scientific CIF names have different roles. Use the Python reference for code and data formats for persistence; identify the actual scientific quantity before comparing a similarly named value.
Comparing calculations¶
Match the structure setting, radiation and scattering sources, instrument calibration, profile definition, background, exclusions, observations and free parameters. Record the software version and minimizer conditions as well.
A comparison is meaningful only for the models and conditions it covers. Retain the complete project and its report with the result.