In a previous talk, “Many-body density matrix perturbation theory“, we introduced a novel method for computing the electron correlation in electronic structure theory using only sparse matrix methods. This linear scaling theory is simple, variational, and highly accurate. However, it is not size extensive, a serious shortcoming of the method. In this talk we show how it is possible to overcome this problem by introducing an exponential correlation operator (as is done in Coupled Cluster theory), which is size extensive by construction. However, this construction leads to an infinite series of diagrams which needs to be summed. We show in this talk how these diagrams can be summed partially which allows us to derive an energy expression which is size extensive.