public LinkedList<Group> getAllHEM(Structure structure) {
        LinkedList<Group> HEMs = new LinkedList<>();
        List<Chain> chains = structure.getChains();
        for (Chain c : chains) {
            for (Group g : c.getAtomGroups()) {
                if (g.getPDBName().equalsIgnoreCase("HEM")) {
                    HEMs.add(g);
                }
            }

        }
        return HEMs;

    }