Bond angle
==========

An angle is defined by three atoms :math:`i,j,k` by

.. math::
    \phi = \cos^{-1}{\left( \
    \frac{r_{ij} \cdot r_{kj}}{ \left| r_{ij} \right|\left| r_{kj} \right|} \
    \right)},

where :math:`r_{ij}` is the connection vector between the atoms :math:`i` and :math:`j`. It takes \
values from 0 to 180 degrees.

The use of the bond angle is invoked by the following block in the `meta-config.json` file::

    "collective variables": [
      {
        "type": "angle",
        "name": "bond angle phi",
        "atoms": [i, j, k]
      },
      ...
    ]

The ``"atoms"`` keyword is used for the definition of the involved atoms in consecutive order.