##############################
Calculating the Beltrami field
##############################

For a given set of interfaces one can calculate the Beltrami field
:math:`\nabla \times \mathbf{B} = \mu \mathbf{B}`, with a set of constraints
specified using the ``lconstraint`` flag. The interfaces used for field
calculation are determined through the ``linitialize`` flag.

The field for a configuration can be found by running the script
``scripts/calc_spectre_field.py`` with a TOML file. The output will be in the
form of a HDF5 file containing the field on a grid, also including the poincare
trace if it is requested in the input file. The output field can be analysed,
as described in the section :doc:`/get_started/postprocessing`

Alternatively, one can create a ``SPECTRE`` object and calculate the interface
force :math:`\mathbf{F}_l = [p + B^2/2\mu_0]_l` which will trigger a field
calculation, as the following code shows:

.. code-block:: python

   from spectre import Spectre, force_real, get_xinit_specwrap

   obj = SPECTRE("file.toml")

   interfaces = get_xinit_specwrap(obj)

   force = force_real(interfaces, obj)