Importance truncation and the further development of the No-Core Shell Model

The No-Core Shell Model, an ab-initio approach to calculating observables of light nuclei, has been shown to be successful in describing p-shell nuclei properties. With the inclusion of the resonating group method, the approach can be extended to describe nuclear reactions. However, the calculations require a large number of basis states for full convergence. By using perturbation theory, one can formulate a procedure, for selecting only those states that one considers ``important.'' This selection procedure is able to drastically reduce the size of the basis, yet captures enough of the physics present, comparing well with full space calculations. I will show calculations in which we calculate the wave functions of Helium-8 (in an importance truncation setting), which are used as input for the n+He-8 scattering calculations. Those theoretical calculations are then compared to recent experiments.