Let us now consider the proper behaviour for matrix elements R pq in the limit of long distance between φ p and φ q . To maintain size-extensivity and size-consistency, the matrix element should vanish for d(p, q) → ∞, where d(p, q) denotes a distance metric between orbitals φ p and φ q . This restriction has a very direct consequence on the construction of regularisation diagrams: 9. The diagram corresponding to R pq â † p âq is connected.