Since the number of all possible permutations interchanging n pairs of electrons between subsystems grows exponentially with n, the total number of permutations in the P operator (leading to non-zero contributions) is very large. This means that the direct inclusion of all the permutations in the calculation of the exchange energy corrections can be extremely costly from a numerical point of view. This fact has led to the emergence and popularity of an approximate scheme involving only permutations of one pair of electrons between the interacting monomers. Such an approach is usually referred to as the single-exchange or S 2 approximation.