Nuclear reaction matrix calculations with a shell-modelQ

The Barrett-Hewitt-McCarthy (BHM) method for calculating the nuclear reaction matrixG is used to compute shell-model matrix elements forA=18 nuclei. The energy denominators in intermediate states containing one unoccupied single-particle (s.p.) state and one valence s.p. state are treated correctly, in contrast to previous calculations. These corrections are not important for valence-shell matrix elements but are found to lead to relatively large changes in cross-shell matrix elements involved in core-polarization diagrams.