where E (10) exch (∝ S 2k ) denotes only these contributions to E (10) exch that are proportional to S 2k , i.e. have exactly k pairs of molecular overlap integrals. Equation ( ) is the well-known expression for S 2 approximation of the first-order exchange energy . First-order exchange energy without intramonomer correlation corrections in S 4 approximation can be written as