@classmethod
def couplings(cls, atom):
    """Generate list of possible couplings for a given atom type.
        
        :param atom: Atom type.
        :type atom: :class:`ctfile.Atom`
        :return: List of couplings.
        :rtype: :py:class:`list`
        """
    return NotImplementedError('Subclass must implement method.')
