Evaluate a water geometry with CPMD
-----------------------------------

This tutorial builds rgpot, loads a CPMD engine, and evaluates one water
geometry.
The geometry file is ``tests/data/cpmd/water.xyz``.
Its title is ``water``.
The atoms are oxygen and two hydrogens, in angstrom, in that order.
The cell is a cube with side 20 angstrom.

What you will need
~~~~~~~~~~~~~~~~~~

- `pixi <https://pixi.sh>`_ (manages the full toolchain)

Clone and build
~~~~~~~~~~~~~~~

.. code:: bash

    git clone https://github.com/OmniPotentRPC/rgpot.git
    cd rgpot
    pixi shell -e rpctest
    meson setup bbdir -Dwith_tests=true -Dwith_rpc=true --buildtype=debug
    meson compile -C bbdir

A tests build produces the stand-in engine and the server:

::

    bbdir/CppCore/libcpmdc_fake_engine.so
    bbdir/CppCore/potserv

Evaluate water
~~~~~~~~~~~~~~

.. code:: bash

    export RGPOT_CPMD_ENGINE="$PWD/bbdir/CppCore/libcpmdc_fake_engine.so"
    python tests/cpmd_water.py \
      --server-bin ./bbdir/CppCore/potserv \
      --expect 0.77

The command prints:

::

    water geometry: water
    water energy: 0.77

``0.77`` is the energy the stand-in engine returns for this geometry and this
cell.
Point ``RGPOT_CPMD_ENGINE`` at a ``libcpmdc.so`` built from the CPMD archive to
evaluate the same file with CPMD.
Read the energy the command prints.

Next steps
~~~~~~~~~~

- :doc:`quickstart`: build, test, and the CuH2 RPC smoke
- :doc:`../reference/mpi`: ``with_mpi`` values ``auto``, ``enabled``, and ``disabled``
- :doc:`../howto/integration`: configure and calculate over RPC