private void recordUnidentifiableModifiedResidues(List<ModifiedCompound> modComps) {
    Set<StructureGroup> identifiedComps = new HashSet<StructureGroup>();
    for (ModifiedCompound mc : modComps) {
      identifiedComps.addAll(mc.getGroups(true));
    }

    // TODO: use the ModifiedAminoAcid after Andreas add that.
    for (Group group : residues) {
      if (group.getType().equals(GroupType.HETATM)) {
        StructureGroup strucGroup = StructureUtil.getStructureGroup(group, true);
        if (!identifiedComps.contains(strucGroup)) {
          unidentifiableModifiedResidues.add(strucGroup);
        }
      }
    }
  }