def METHOD_NAME(self, start=None, unit='angstrom'):
    """Return a vector between two atoms that form this bond. *start* can be used to indicate which atom should be the beginning of that vector. If not specified, ``self.atom1`` is used. Returned value if a tuple of length 3, expressed in *unit*.
    """
    if start:
        if start not in self:
            raise MoleculeError('Bond.as_vector: given atom is not a part of this bond')
        a,b = start, self.other_end(start)
    else:
        a,b = self.atom1, self.atom2
    return a.vector_to(b, result_unit=unit)