Average characteristics of the configuration interaction in atoms and their applications

• We consider average characteristics of configuration interaction in atoms.
• The explicit expressions for the main average characteristics are presented.
• Such characteristics present the useful method for the selection of wave function basis.
• These characteristics enable to estimate correlation effects in atoms.

The average characteristics of the configuration interaction (CI) present the perspective, but yet little used method for the estimation and investigation of CI effects in atoms. This work is the first attempt to give the systematic view on such characteristics. Their general, but rather simple explicit expressions are presented. The application of these characteristics for the investigation of CI effects in atoms is illustrated by calculation results for practically important cases of configurations with a symmetric exchange of symmetry, (sd)N complex and some ground configurations. It is shown that the average weight of one configuration in the expansion of the wave functions of the other configuration as well the average shift of energy level of configuration due to its interaction with the levels of distant configuration present useful characteristics for the selection of wave function basis. The average characteristics can also be applied for the estimation of CI regularities in the isoelectronic and isonuclear sequences, the determination of the types of CI.