The In-Medium No-Core Shell Model as Comprehensive Ab-Initio Tool

In this work we apply and extend the in-medium no-core shell model (IM-NCSM), which is a combination of the multi-reference in-medium similarity renormalization group (IM-SRG) and the no-core shell model (NCSM). The IM-NCSM contains the best features of both methods, i.e., the exact diagonalization of the NCSM giving access to ground-state and excited-state properties and the multi-reference state decoupling of the IM-SRG accelerating the convergence of the NCSM diagonalization. Therefore, the IM-NCSM is a very powerful method to investigate properties of open-shell medium-mass nuclei in the sd-shell and beyond. We have used the IM-NCSM to compute scalar and non-scalar observables for a large range of even and odd isotopes in the oxygen, carbon and neon isotopic chains with a new family of chiral interactions consisting of nucleon-nucleon (NN) interactions by Entem, Machleidt and Nosyk and three-nucleon (3N) interactions by Hüther et al. This includes a full estimation of many-body and interaction uncertainties. The latter are estimated using a model based on Bayesian statistics. With these interactions we obtain converged results for ground-state energies, spectra, charge radii, and electromagnetic observables for even and odd isotopes of the aforementioned isotopic chains. In comparison with experimental results the new family of interactions gives good results for all scalar observables, even for the fully open-shell neon isotopes. Since we systematically underestimate electric quadrupole transition strengths of oxygen and neon isotopes we extended the IM-NCSM by a leading-order three-body correction (LOTC) for the inclusion of induced three-body terms. This correction alone does not solve the underlying problem of underestimated transition strengths, but it can be used as a tool to obtain an error estimate when neglecting induced three-body contributions. With the LOTC a corner stone for future research in the IM-NCSM is set.