Large-scale computation of the no-core Monte Carlo shell model for nuclear many-body problems*

With the aid of recent progress on computational environments and nuclear many-body calculation techniques adapted to large-scale computations on the state-of-art supercomputers, ab-initio approaches for nuclear structure and reactions have greatly advanced and are now providing first principles results for low-energy nuclear theory. As one of these ab-initio methods for nuclear structure, the no-core Monte Carlo shell model (MCSM) has also been developed in recent years, following the successes in the standard MCSM calculations with shell-model effective interactions under the assumption of the inert core and valence space. In this contribution, we will briefly summarize the current status of no-core MCSM calculations. No-core MCSM results in light nuclei with realistic two-nucleon forces, JISP16 and Daejeon16 NN interactions, will be given as one of the demonstrations for the capability of our method. From the current understanding of ab-initio theory, three-nucleon forces (3NFs) are indispensable for ab-initio calculations to reproduce experimental data. Concerning the inclusion of 3NF effects, the no-core MCSM calculations with similarity-renormalization-group-evolved chiral effective-field-theory interactions will be presented so as to investigate the 3NF effects on calculated observables. Our attempts for the understanding of alpha-cluster and molecular-orbital structure in terms of nucleon degrees of freedom will also be mentioned, focusing on the Be and C isotopes. The intrinsic shapes obtained from no-core MCSM wavefunctions without any assumptions of alpha-cluster and molecular-orbital structure are qualitatively consistent with those from the model calculations.