Zero-order approximation probability density

In other words, if we assume that the counting function N(t) has statistically independent increments (Eq. (3-237)), and has the property that the probability of a single jump occurring in a small interval of length h is approximately nh but the probability of more than one jump is zero to within terms of order h, (Eq. (3-238)), then it can be shown 51 that its probability density functions must be given by Eq. (3-231). It is the existence of theorems of this type that accounts for the great... 

What about the exchange contribution This contribution does not exist in the polarization approximation. It appears only in symmetry-adapted perturbation theory, in pure form in the first-order energy correction and coupled to other effects in higher order energy corrections. The exchange interaction is difficult to interpret, because it appears as a result of the antisymmetry of the wave fimction (Pauli exclusion principle). The antisymmetry is forced by one of the postulates of quantum mechanics (see Chapter 1) and its immediate consequence is that the probability density of finding two electrons with the same spin and space coordinates is equal to zero. 

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