NOON
====

The use of a natural orbital occupation number (NOON) as CV is invoked by the following block in
the `meta-config.json` file::

    "collective variables": [
      {
        "type": "noon",
        "name": "natural orbital occupation",
        "no_index": i,
        "nproc": 1
      },
      ...
    ]

``"no_index"`` is the keyword for the desired orbital. Orbital counting starts at 0.
Gradients are calculated numerically. This is step is parallelized using the :mod:`multiprocessing`
python package. The number of used processors is specified with the ``"nproc"`` keyword.
Currently, only the :doc:`Molpro <../interfaces/molpro>` calculator interface can be used in
combination with this CV and it has to provide natural orbital occupation numbers. For this purpose,
Molpro has to print an xml file with the name `natural.xml` containing the natural orbital
information. Include the following code in your Molpro tail file::

    put,xml,natural.xml;orb,2141.2

`2141.2` is the directive in the Molpro wavefunction file where the natural orbital information is
stored.