By taking into consideration all the limitations imposed on the correct diagrams one can find a few types of diagrams that satisfy them. Then, by determining possible places where another pair of S-integrals can be inserted, diagrams of a higher order of overlap, i.e. S 2(n+1) , can be generated. Considering all possible types of diagrams and ways to increase their order in the overlap matrix elements S, one can write expressions for the firstorder exchange energy without intramonomer correlation corrections in terms of one-and two-electron integrals. Assuming, that the RHF method was used to obtain the reference determinants, the expression takes form