Energy gap
==========

The energy gap between two electronic states :math:`i,j` is defined by the difference of their energies

.. math::
    \Delta E = E_j-E_i

The use of the energy gap is invoked by the following block in the `meta-config.json` \
file::

    "collective variables": [
      {
        "type": "energygap",
        "name": "energy gap between two states",
        "state1": i,
        "state2": j,
        "do_multistate": False
      },
      ...
    ]

The ``"state1"`` and ``"state2"`` keywords set the electronic state indices to be used. This requires \
that the calculator provides energies and gradients for both states. Currently, the `energygap` \
CV can only be used in combination with the :doc:`Molpro <../interfaces/molpro>` interface.

The Multistate Metadynamics extension is used, when ``"do_multistate"`` is set to ``True``. In this \
case, the configuration for a second metadynamics instance has to be provided under the \
``"metadynamics_multistate"`` keyword in the `meta-config.json` file.