Which, however, is not to say that ATK cannot be used for your system. What the referee asks, is how do you know that your system is not weakly coupled. A simple answer is that you assume that it isn't... A more complete answer, which probably might be needed, can only be found by you - nobody else knows what your system even looks like. By providing such an answer you will show the referee that you understand what you are studying. This doesn't mean that you need to understand all the basic formalism that ATK uses, except as zh writes, that ATK assume the system is bonded, and not weakly coupled.