ABSTRACT
In preceding sections, examples of rather pure configurations were given. It was explained that pure shell model configurations may be used for the calculation of energies only if the effect of other configurations may be implicitly included as renormalization of the effective interaction. Configuration mixings were shown to be explicitly important in some cases but they were treated rather simply, in the formalism of generalized seniority. In many cases, however, there are clear effects of configuration mixing which cannot be replaced by simple renormalization and more detailed calculations must be carried out. Mixing of configurations has been considered in the shell model following two different directions. One way to deal with configuration mixing is to construct the shell model Hamiltonian in a subspace defined by all configurations involved. That submatrix of the Hamiltonian is then diagonalized. It is possible to carry out this procedure only in cases where the order of the matrices is manageable. This procedure has been adopted for nucleons occupying a rather small number of j-orbits whose single nucleon energies are close. The most complicated yet successful program has been carried out by Wildenthal et al. (1984) in the 1d 5/2, 2s 1/2, 1d 3/2 shell.
