code: public static double getTotalNaturalAbundance(IMolecularFormula formula) {
          double abundance = 1.0;
          for (IIsotope isotope : formula.isotopes()) {
              if (isotope.getNaturalAbundance() == null) return 0.0;
              abundance = abundance * Math.pow(isotope.getNaturalAbundance(), formula.getIsotopeCount(isotope));
          }
          return abundance / Math.pow(100, getAtomCount(formula));
      }
docs: Get the summed natural abundance of all isotopes from an MolecularFormula. Assumes
  abundances to be preset, and will return 0.0 if not.
  
  @param  formula The IMolecularFormula to calculate
  @return         The summed natural abundance of all isotopes in this MolecularFormula