We perform ab initio no core shell model calculations for A = 18 and 19 nuclei in a 4hΩ, or Nmax = 4, model space using the effective JISP16 and chiral N3LO nucleon-nucleon potentials and transform the many-body effective Hamiltonians into the 0hΩ model space to construct the A-body effective Hamiltonians in the sd-shell. We separate the A-body effective Hamiltonians with A = 18 and A = 19 into inert core, one-and two-body components. Then, we use these core, one-and two-body components to perform standard shell model calculations for the A = 18 and A = 19 systems with valence nucleons restricted to the sd-shell. Finally, we compare the standard shell model results in the 0hΩ model space with the exact no core shell model results in the 4hΩ model space for the A = 18 and A = 19 systems and find good agreement.

We perform a quantitative study of the microscopic effective shell-model interactions in the valence sd shell, obtained from modern nucleon-nucleon potentials, chiral N3LO, JISP16 and Daejeon16, using No-Core Shell-Model wave functions and the Okubo-Lee-Suzuki transformation. We investigate the monopole properties of those interactions in comparison with the phenomenological universal sd-shell interaction, USDB. Theoretical binding energies and low-energy spectra of O isotopes and of selected sd-shell nuclei, are presented. We conclude that there is a noticeable improvement in the quality of the effective interaction when it is derived from the Daejeon16 potential. We show that its proton-neutron centroids are consistent with those from USDB. We then propose monopole modifications of the Daejeon16 centroids in order to provide an adjusted interaction yielding significantly improved agreement with the experiment. A spin-tensor decomposition of two-body effective interactions is applied in order to extract more information on the structure of the centroids and to understand the reason for deficiencies arising from our current theoretical approximations. The issue of the possible role of the three-nucleon forces is addressed. nificant progress in ab-initio calculations for light nuclei, the structure of the open-shell medium-mass nuclei can still be described only by restricted valence-space calculations. Thus, the goal to derive effective valence-space interactions represents a major area of endeavor.The present study is focused on the use of the No-Core Shell Model (NCSM) [1] in conjunction with additional theoretical treatment that leads to effective interactions for valence space shell-model calculations. Within the NCSM, all A nucleons, interacting via realistic forces, are treated as active within a model space, consisting of a large number of shells (typically, shells of a harmonicoscillator potential). The eigenvalue problem is solved by diagonalization of the many-body Hamiltonian matrix in a spherically symmetric harmonic-oscillator basis. The many-body eigenstates, represented as mixing of configurations expressed as Slater determinants of the proton and neutron single-particle wave functions, preserve all fundamental symmetries of atomic nuclei and can be used directly to calculate matrix elements of various operators. As a fully ab-initio approach, the NCSM

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