Torsion angle
=============

A torsion angle is defined by four atoms :math:`i,j,k,l` by

.. math::
    \theta = \sphericalangle \left( P_{ijk},P_{jkl} \right)

where :math:`P_{ijk}` and :math:`P_{jkl}` are planes determined by the atoms :math:`i,j,k` and \
:math:`j,k,l`, respectively. It takes values from -180 to 180 degrees.

The use of the torsion angle is invoked by the following block in the `meta-config.json` file. The \
``"atoms"`` keyword is used for the definition of the involved atoms in consecutive order::

    "collective variables": [
      {
        "type": "torsion",
        "name": "torsion angle theta",
        "atoms": [i, j, k, l]
      },
      ...
    ]