def determine_approximant_to_use(template_params):
    """Decision tree to determine the which approximant to use

    Parameters
    ----------
    template_params: dict
        dictionary of the best matching template parameters

    Returns
    -------
    approximant: str
        the approximant name
    """
    mass_ratio = float(template_params['mass_1']) / float(template_params[
        'mass_2'])
    if mass_ratio < 18:
        approximant = 'IMRPhenomD'
    else:
        approximant = 'SEOBNRv4'
    return approximant
