Assigns force field parameters to Atoms in the AMPAL object.

        Parameters
        ----------
        ff: BuffForceField
            The force field to be used for scoring.
        mol2: bool, optional
            If true, mol2 style labels will also be used.

def assign_force_field(self, ff, mol2=False):
        """Assigns force field parameters to Atoms in the AMPAL object.

        Parameters
        ----------
        ff: BuffForceField
            The force field to be used for scoring.
        mol2: bool, optional
            If true, mol2 style labels will also be used.
        """
        if hasattr(self, 'ligands'):
            atoms = self.get_atoms(ligands=True, inc_alt_states=True)
        else:
            atoms = self.get_atoms(inc_alt_states=True)
        for atom in atoms:
            w_str = None
            a_ff_id = None
            if atom.element == 'H':
                continue
            elif atom.ampal_parent.mol_code in ff:
                if atom.res_label in ff[atom.ampal_parent.mol_code]:
                    a_ff_id = (atom.ampal_parent.mol_code, atom.res_label)
                elif atom.res_label in ff['WLD']:
                    a_ff_id = ('WLD', atom.res_label)
                else:
                    w_str = ('{} atom is not parameterised in the selected '
                             'force field for {} residues, this will be '
                             'ignored.').format(
                        atom.res_label, atom.ampal_parent.mol_code)
            elif atom.res_label in ff['WLD']:
                a_ff_id = ('WLD', atom.res_label)
            elif mol2 and (atom.ampal_parent.mol_code.capitalize() in ff['MOL2']):
                a_ff_id = ('MOL2', atom.res_label.capitalize())
            else:
                if not mol2:
                    w_str = ('{} ({}) atom is not parameterised in the selected'
                             ' residue force field. Try activating the heavy '
                             ' atom force field (haff).').format(
                        atom.element, atom.res_label)
                else:
                    w_str = ('{} ({}) atom is not parameterised in the selected'
                             ' force field.').format(atom.element, atom.res_label)
            if w_str:
                warnings.warn(w_str, NotParameterisedWarning)
            atom._ff_id = a_ff_id
        self.tags['assigned_ff'] = True
        return