Gaussian Backend
================

Gaussian is a quantum-chemistry backend available through PyAR for users who
have a licensed Gaussian installation and want higher-level electronic-
structure calculations.

When to use Gaussian
--------------------

Use ``gaussian`` when:

* Gaussian is installed and accessible in your environment
* you want DFT-style optimisation or refinement
* you are refining a smaller set of candidates generated by cheaper PyAR
  searches
* you have validated a small test calculation with your method, basis, charge,
  and multiplicity

Example command
---------------

Reaction search using the registered Gaussian energy-gradient route:

.. code-block:: bash

   pyar-cli react A.xyz B.xyz -N 8 -gmin 100 -gmax 1000 --software gaussian --method B3LYP --basis 6-31G*

Practical notes
---------------

* Gaussian must be installed separately and configured according to your local
  licence and system setup.
* Confirm the executable name and output handling on a small molecule before
  running larger PyAR jobs.
* Gaussian calculations are usually much slower than xTB or ML-potential runs.
  For screening, use xTB/AIMNet2 first and refine selected structures later.

AFIR/geomeTRIC status
---------------------

``gaussian`` is wired as a registered energy-gradient provider for the
geomeTRIC-backed AFIR reaction route. This means PyAR can construct a combined
``Gaussian energy/gradient + AFIR bias`` objective for geomeTRIC. Local
validation is still required because Gaussian setup and output handling depend
on the installed executable and environment.