def tag_atoms(mol, atoms_to_tag, tag="x"):
    """Tag atoms with a specified string

    :param mol: input molecule
    :param atoms_to_tag: indices of atoms to tag
    :param tag: string to use for the tags
    :return: molecule with atoms tagged
    """
    [atm.SetProp('atomNote', "") for atm in mol.GetAtoms()]
    [mol.GetAtomWithIdx(idx).SetProp('atomNote', tag) for idx in atoms_to_tag]
    return mol