Fitting Tab
===========

The **Fitting** tab provides a centralized view and control over all parameters
that enter the R-matrix fit. It allows setting initial values, parameter limits,
and nuisance parameter flags.

Parameter Tables
----------------

Parameters are organized into three sub-tabs:

Level Parameters
^^^^^^^^^^^^^^^^

Displays one row for each level-channel combination with the following columns:

.. list-table::
   :widths: 20 80
   :header-rows: 1

   * - Column
     - Description
   * - **Name**
     - Identifier for the parameter (level energy or channel width).
   * - **Value**
     - Current parameter value (editable).
   * - **Lower Limit**
     - Lower bound for the parameter during fitting (leave empty for no bound).
   * - **Upper Limit**
     - Upper bound for the parameter during fitting (leave empty for no bound).
   * - **Error**
     - Prior uncertainty for use as a nuisance parameter.
   * - **Fit Error**
     - Uncertainty from the fit result (read-only, populated after fitting).
   * - **Use as Nuisance**
     - Check to treat this parameter as a nuisance parameter with a Gaussian
       prior in the chi-squared calculation.

Normalization Parameters
^^^^^^^^^^^^^^^^^^^^^^^^

Lists the normalization factors for each data segment that has **Vary Norm?**
enabled. These parameters scale the data yield during fitting.

Energy Shift Parameters
^^^^^^^^^^^^^^^^^^^^^^^^

Lists the energy shift parameters for each data segment that has **Vary Energy
Shift?** enabled.

Controls
--------

Refresh from Current
   Rebuild the parameter table from the current GUI state (particle pairs,
   levels, segments).

Load from .sav File
   Import parameters from a ``param.sav`` file generated by a previous fit.
   The normalization file (``normalizations.out``) from the same directory is
   loaded automatically.

Populate Wigner Limits
   Automatically set upper and lower limits based on the Wigner limit
   (single-particle limit) for reduced width amplitudes. This provides
   physically motivated bounds for the fit.

Clear Limits
   Remove all upper and lower limit constraints from all parameters.

.. tip::

   Using the **Populate Wigner Limits** feature is a good starting point for
   constraining fits, especially for initial explorations of parameter space.
   Limits can then be adjusted manually as needed.