Clearly, there are still infinitely many diagrams that can be constructed which fulfill all of the established rules. Since we have introduced no restriction on the size of these diagrams so far, many are prohibitively expensive to evaluate. With the practical utility of the resulting second order perturbation theory in mind, we add further constraints which limit the computational scaling of each contribution: 6. As a function of molecule size, the computational scaling does not exceed O(M 5 ).