Coulomb exchange interaction between electrons in the atom and structure of complex configurations

The Coulomb exchange interaction operator and its matrix elements are represented in a special form, which enables the investigation of groups of terms, caused by this interaction, and the construction of new wavefunction bases. The existence of distinct upper and lower groups of terms due to exchange interaction in configurations n(l-1) -1 nl N and nl 4l+2-N n(l+1) for small numbers of electrons or vacancies N=1, 2 is shown. The separation of groups becomes more pronounced in the two-configuration approximation on taking into account correlations with symmetric exchange of symmetry. In this case a new wavefunction basis is proposed.