def get_reaction_label(rmg_reaction):
    """
    Returns the AutoTST reaction string in the form of r1+r2_p1+p2 (e.g., `CCC+[O]O_[CH2]CC+OO`).
    `reactants` and `products` are lists of class:`Molecule`s.
    """
    reactants = rmg_reaction.reactants
    products = rmg_reaction.products
    if len(reactants) > 1:
        reactants_string = '+'.join([reactant.molecule[0].toSMILES() for reactant in reactants])
    else:
        reactants_string = reactants[0].molecule[0].toSMILES()
    if len(products) > 1:
        products_string = '+'.join([product.molecule[0].toSMILES() for product in products])
    else:
        products_string = products[0].molecule[0].toSMILES()
    reaction_label = '_'.join([reactants_string, products_string])
    return reaction_label