code: private static double calculateExactMass(List<AbstractMolecule> molecules) throws CTKException, ChemistryException, BuilderMoleculeException {
      Double result = 0.0;
      for (AbstractMolecule molecule : molecules) {
          molecule = BuilderMolecule.mergeRgroups(molecule);
        result += Chemistry.getInstance().getManipulator().getMoleculeInfo(molecule).getExactMass();
      }
      return result;
    }
docs: intern method to calculate the exact mass for a list of molecules
  
  @param molecules
  @return
  @throws CTKException
  @throws ChemistryException if the Chemistry Engine can not be initialized
  @throws BuilderMoleculeException