A simple model for the continuum-continuum interaction

An approximate method for calculating the effects of the continuum-continuum interaction which arises in shell-model reaction theories is presented and applied to the dipole photoabsorption cross section on 16O in the Tamm-Dancoff approximation. The approximation consists of fitting analytic functions to selected bound-continuum matrix elements and utilizing these functions to calculate continuum-continuum and the remaining bound-continuum matrix elements. (Comparisons with selected exact calculations indicate at worst 20 % error.) Using this technique, we find an upward shift in the position of the giant dipole resonance peaks and a decrease in the widths, leading to a 40 % overall diminution in the total integrated cross section.