code: public IAtomType getAtomType(String identifier) throws NoSuchAtomTypeException {
      	IAtomType type = atomTypes.get(identifier);
      	if (type != null) return type;
          throw new NoSuchAtomTypeException("The AtomType " + identifier + " could not be found");
      }
docs: Get an AtomType with the given ID.
  
  @param  identifier                   an ID for a particular atom type (like C$)
  @return                              The AtomType for this id
  @exception  NoSuchAtomTypeException  Thrown if the atom type does not exist.