3D Wiener number
================

The 3-dimensional Wiener number is defined by the sum over all interatomic distances

.. math::
    ^{3D}W = \frac{1}{2} \sum_i^N \sum_j^N d_{ij}.

For a given molecule, small values of :math:`^{3D}W` correspond to a compact, spherical-like \
overall shape, while large values indicate an expanded structure.

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

    "collective variables": [
      {
        "type": "wiener",
        "name": "3d wiener number",
        "exclude_h": False
      },
      ...
    ]

The additional keyword ``"exclude_h"`` allows for the calculation of the Wiener number without \
considering distances from and to Hydrogen atoms.