Size consistency

In quantum chemistry, size consistency (or strict separability) is a property that guarantees the consistency of the energy behavior when interaction between the involved molecular system is nullified (for example, by distance).

Let A and B be two non-interacting systems. If a given theory for the evaluation of the energy is size consistent, then the energy of the supersystem A-B is equal to the sum of the energy of A plus the energy of B taken by themselves ($$E(A-B) = E(A) + E(B)$$). This property is of particular importance to obtain correctly behaving dissociation curves.

Both variational and perturbative methods can have size consistency issues. Various solutions have been developed to reduce or counterbalance the non-size consistent behavior. However, particular choices of the theoretical deployment can guarantee a size consistent behavior of the approach, like in the case of the coupled cluster method.