public static Group getHEM(Structure structure) {
        List<Chain> chains = structure.getChains();
        for (Chain c : chains) {
            for (Group g : c.getAtomGroups()) {
                if (g.getPDBName().equalsIgnoreCase("HEM")) {
//                    System.out.println(c.getName());
//                    System.out.println(g.getChainId());
                    return g;
                }
            }

        }
        return null;

    }