def distance_based_on_molecular_formula(metabolite1, metabolite2, normalize=True):
    """Calculate the distance of two metabolites bases on the molecular formula

    Arguments
    ---------
    metabolite1 : Metabolite
        The first metabolite.
    metabolite2 : Metabolite
        The second metabolite.
    normalize : bool, optional
        If the distance should be normalized by the total number of elements in both metabolites (defaults to True).

    Returns
    -------
    float
        The distance between metabolite1 and metabolite2.
    """
    if len(metabolite1.elements) == 0 or len(metabolite2.elements) == 0:
        raise ValueError('Cannot calculate distance between metabolites %s and %s' % (metabolite1, metabolite2))
    elements = set(list(metabolite1.elements.keys()) + list(metabolite2.elements.keys()))
    distance = 0.
    for element in elements:
        distance += abs(metabolite1.elements.get(element, 0) - metabolite2.elements.get(element, 0))
    if normalize:
        return distance / sum(list(metabolite1.elements.values()) + list(metabolite2.elements.values()))
    else:
        return distance