Multi-reference energy density functional calculations : status, challenges and perspectives
A key challenge for ab-initio theory is to describe and predict properties of medium mass nuclei from the valley of stability towards the driplines, especially in relation to the wealth of new experimental data now coming from radioactive beam facilities. The nuclear many-body problem is a difficult undertaking from both the computational and theoretical points of view. Techniques such as Green's function Monte Carlo (GFMC) and no-core shell model (NCSM) allow essentially exact calculations, but are limited to light nuclei. For mid-mass isotopes above A=16, the challenge posed by the numerical scaling demands innovative many-body theory techniques and computational approaches. This is especially true for the extensions to nuclei with an open-shell character. Techniques such as self-consistent Green's function (SCGF), coupled cluster (CC), in-medium similarity renormalization group (IMSRG) or importance truncated no-core shell model (IT-NCSM) are promising methods due to relatively soft scaling with particle number. Current implementations of these methods are able to describe nuclei in the immediate vicinity of (sub-)closed shells up to the Nickel's mass regions. The current frontier consists of extending such calculations to higher masses and developing extensions to truly open-shell nuclei, and describing structure and reaction properties in a consistent ab-initio framework starting from modern 2N and 3N interactions. Continued progress in these areas will open the possibility of theoretical predictions for a wealth of processes that were previously inaccessible to microscopic methods. In this sense, we are about to witness a major turning point in low-energy nuclear physics.