It is possible to state the first-order exchange energy, exact in the overlap expansion, as a series of corrections of given orders of S, i.e. by extending the upper bound of summation in the right-hand side of the equation (39) to infinity. It is stated here as a conjecture that each E (10) exch (∝ S 2n ) contribution can be obtained as a linked part of the V P2n term. The reasoning behind this assumption is the following. Consider the form of individual terms in the equation (35), it can be easily seen that in each order only the V P2n term contains linked contributions. The remaining terms have only unlinked-diagram contributions, as they are products of expectation values, which cancel out the unlinked contributions from the first term. This cancellation is anticipated in analogy to the linked-diagram theorem holding for Rayleigh-Schrödinger perturbation theory. Although not rigorously proven, the used conjecture has been verified to be valid up to terms with the P8 operator using our symbolic algebra program. Hence one should be able to write first-order exchange energy without intramonomer corrections as