Set the charge and spin multiplicity.

        Args:
            charge (int): Charge for the molecule. Defaults to 0.
            spin_multiplicity (int): Spin multiplicity for molecule.
                Defaults to None, which means that the spin multiplicity is
                set to 1 if the molecule has no unpaired electrons and to 2
                if there are unpaired electrons.

def set_charge_and_spin(self, charge, spin_multiplicity=None):
        """
        Set the charge and spin multiplicity.

        Args:
            charge (int): Charge for the molecule. Defaults to 0.
            spin_multiplicity (int): Spin multiplicity for molecule.
                Defaults to None, which means that the spin multiplicity is
                set to 1 if the molecule has no unpaired electrons and to 2
                if there are unpaired electrons.
        """
        self._charge = charge
        nelectrons = 0
        for site in self._sites:
            for sp, amt in site.species.items():
                if not isinstance(sp, DummySpecie):
                    nelectrons += sp.Z * amt
        nelectrons -= charge
        self._nelectrons = nelectrons
        if spin_multiplicity:
            if (nelectrons + spin_multiplicity) % 2 != 1:
                raise ValueError(
                    "Charge of {} and spin multiplicity of {} is"
                    " not possible for this molecule".format(
                        self._charge, spin_multiplicity))
            self._spin_multiplicity = spin_multiplicity
        else:
            self._spin_multiplicity = 1 if nelectrons % 2 == 0 else 2