def METHOD_NAME(self, a):
    """Return neighbors of atom number a.
    A list of indices to neighboring atoms is
    returned.
    Parameters
    ----------
    a : int
        atomic index
    Returns
    -------
    np.array
        array of neighbouring indices
    Raises
    ------
    RuntimeError
        Must update the list at least once!
    """
    if self.nupdates == 0:
        raise RuntimeError("Must update the list at least once!")
    return self.neighbours[a]