We use the recently proposed In-Medium Similarity Renormalization Group (IM-SRG) to carry out a systematic study of closed-shell nuclei up to 56 Ni, based on chiral two-plus three-nucleon interactions. We analyze the capabilities of the IM-SRG by comparing our results for the groundstate energy to Coupled Cluster calculations, as well as to quasi-exact results from the ImportanceTruncated No-Core Shell Model. Using chiral two-plus three-nucleon Hamiltonians whose resolution scales are lowered by free-space SRG evolution, we obtain good agreement with experimental binding energies in 4 He and the closed-shell oxygen isotopes, while the calcium and nickel isotopes are somewhat overbound.

We formulate the in-medium similarity renormalization group (IM-SRG) for open-shell nuclei using a multireference formalism based on a generalized Wick theorem introduced in quantum chemistry. The resulting multireference IM-SRG (MR-IM-SRG) is used to perform the first ab initio study of all even oxygen isotopes with chiral nucleon-nucleon and three-nucleon interactions, from the proton to the neutron drip lines. We obtain an excellent reproduction of experimental ground-state energies with quantified uncertainties, which is validated by results from the importance-truncated no-core shell model and the coupled cluster method. The agreement between conceptually different many-body approaches and experiment highlights the predictive power of current chiral two- and three-nucleon interactions, and establishes the MR-IM-SRG as a promising new tool for ab initio calculations of medium-mass nuclei far from shell closures.

We present the first ab initio construction of valence-space Hamiltonians for medium-mass nuclei based on chiral two-and three-nucleon interactions using the in-medium similarity renormalization group. When applied to the oxygen isotopes, we find experimental ground-state energies are well reproduced, including the flat trend beyond the drip line at 24 O. Similarly, natural-parity spectra in 21,22,23,24 O are in agreement with experiment, and we present predictions for excited states in 25,26 O. The results exhibit a weak dependence on the harmonic-oscillator (HO) basis parameter and reproduce spectroscopy within the standard sd valence space.PACS numbers: 21.30. Fe, 21.60.Cs, 21.60.De, With the next generation of rare-isotope beam facilities, the quest to discover and understand the properties of exotic nuclei from first principles is a fundamental challenge for nuclear theory. This challenge is complicated in part because the proper inclusion of three-nucleon (3N) forces plays a decisive role in determining the structure of medium-mass nuclei [1,2]. While ab initio many-body methods based on nuclear forces from chiral effective field theory (EFT) [3][4][5] have now reached the medium-mass region and beyond [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], restrictions in the nuclei and observables accessible to these methods have limited their application primarily to ground-state properties in semimagic isotopic chains.For open-shell systems, rather than solving the full Abody problem, it is profitable to follow the shell-model paradigm by constructing and diagonalizing an effective Hamiltonian in which the active degrees of freedom are A v valence nucleons confined to a few orbitals near the Fermi level. Both phenomenological and microscopic implementations of the shell model have been used with success to understand and predict the evolution of shell structure, properties of ground and excited states, and electroweak transitions [21][22][23].Recent microscopic shell-model studies have revealed the impact of 3N forces in predicting ground-and excited-state properties in neutron-and proton-rich nuclei [1,2,[24][25][26][27][28]. Despite the novel insights gained from these studies, they make approximations which are difficult to benchmark. The microscopic derivation of the effective valence-space Hamiltonian relies on many-body perturbation theory (MBPT) [29], where order-by-order convergence is unclear. Even with efforts to calculate particular classes of diagrams nonperturbatively [30], results are sensitive to the HO frequency ω (due to the core), and the choice of valence space [2,24,25]. A nonperturbative method to address these issues was developed in [31,32], which generates valence-space interactions and operators by projecting their full no-core shell model (NCSM) counterparts into a given valence space.To overcome these limitations in heavier systems, the in-medium similarity renormalization group (IM-SRG), originally developed for ab initio calculations of ground states in closed-shell syste...