Ab initio shell model with a core: Extending the No Core Shell Model to heavier nuclei

The No Core Shell Model (NCSM) is an ab initio method for calculating the properties of atomic nuclei, starting with the fundamental interactions among the nucleons and treating all A nucleons as being active. This approach has been successfully applied to nuclei with A ≤ 20, but it is difficult to treat heavier mass nuclei with existing computer technology, because of the extemely large basis spaces required to obtain converged results. In this presentation we outline a new formalism for extending the NCSM to heavier mass nuclei. It involves using the NCSM approach to determine the core, one-body and two-body (and perhaps also three-body) terms, which are the usual input for standard shell model (SSM) calculations. Such SSM calculations can be easily performed for sd- and pf-shell nuclei. As a test of this new formalism, we apply it to nuclei in the 0p-shell, for which exact NCSM calculations can also be performed for making comparisons. Results are given both for energy spectra and electromagnetic properties.