First-principles techniques, such as the No-Core Shell Model (NCSM) or Green's function Monte-Carlo (GFMC), have demonstrated that nuclear structure can be calculated to a very high degree of precision for A<16, when the appropriate two- and three-nucleon forces are included. However, in the case of the NCSM, the calculations require a large number of basis states for full convergence. By using a perturbation theory argument, 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 model-space calculations. I will present an in depth error analysis of importance truncation in the NCSM, and explain how I determine the error of the calculation, using Lithium-6 as a test case. As an application of Importance truncated NCSM, I will calculate the wavefunctions of Helium-8, which are used later in a NCSM/RGM setting; an ab-initio technique for dealing with light-ion reactions. The NCSM/RGM calculations will attempt to determine if Helium-9 is bound or not, by analysing the neutron+Helium-8 scattering process. Furthermore, we can compare our theoretical calculations directly to two conflicting experimental results.
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