Parameters
==========

GASFIR uses a small set of dimensionless parameters to characterise the
ionizing medium. They are stored in a JSON-backed parameter store that ships
with the package.

Built-in Parameter Sets
-----------------------

Retrieve a list of all available names::

    from gasfir import get_parameters
    print(get_parameters())

Load a specific set::

    params = get_parameters("Hydrogen_SFA")
    # {'E_g': 0.5, 'a0': ..., 'a1': ..., 'a2': ..., 'a3': ..., 'a4': 0}

Parameter Descriptions
----------------------

.. list-table::
   :header-rows: 1
   :widths: 10 90

   * - Name
     - Description
   * - ``E_g``
     - Band gap / ionization potential (a.u.)
   * - ``a0``
     - Normalization prefactor
   * - ``a1``
     - Tunneling time / Gaussian smearing parameter
   * - ``a2``
     - Static Stark shift correction
   * - ``a3``
     - Dynamic Stark shift correction
   * - ``a4``
     - Coulomb correction exponent
   * - ``m_eff``
     - Effective mass (for solid-state systems; default 1.0)

Adding Custom Parameters
------------------------

Persist a new parameter set to the on-disk store::

    from gasfir import _save_parameters

    _save_parameters("MyMolecule", {
        "E_g": 0.45, "a0": 5.0, "a1": 3.0,
        "a2": 2.5, "a3": 1.0, "a4": -0.5,
    })

The entry is written to ``src/gasfir/data/parameters.json`` and is available
in future sessions.