The average energy distance between two interacting configurations and itsapplication for the investigation of Cl in atoms

The general explicit expressions for the average energy distance between tw o configurations using the squares of interconfiguration matrix elements as weight coefficients are obtained. Two types of CI are investigated conside ring this quantity: configurations with all levels equally influencing the levels of the other configuration and the configurations with their upper l evels mainly interacting with those of the other configuration. It is demon strated that the obtained energy distance enables to estimate the averaged correlation correction and mixing weights rather accurately and thus can be useful for the selection of sets of admixed configurations and for other a pplications.