Not much later, van Duijneveldt-van de Rijdt and van Duijneveldt extended the approach of Murrel and coworkers to include effects arising from the exchange of two pairs of electrons, leading to expressions for the first-order interaction energy that are exact up to the fourth power of the overlap S integrals. In 1976, Jeziorski et al. derived the expression for the first-order exchange energy, which is entirely accurate in all orders of the overlap expansion. The formula presented in the article has a closed form and is exact for the case of single-determinant reference wavefunctions of isolated monomers. It was derived using one-and two-electron density matrices and cofactors of S matrix -the matrix of overlap integrals.