Ab Initio Shell Model With A Core

The No Core Shell Model (NCSM) has been successful in describing the properties of light nuclei, A <= 16, starting from the fundamental interactions among the A nucleons, but it is currently difficult to extend the NCSM to heavier nuclei, because of the extremely large model spaces involved in the calculations. We present a new procedure, based on performing two unitary transformations on the Hamiltonian, so as to obtain the core, one-body, two-body and perhaps three-body input necessary for performing Standard Shell Model (SSM) calculations for nuclei. Such SSM calculations can be easily performed for sd- and pf-shell nuclei. We demonstrate the accuracy of our approach by applying to it nuclei within the Op-shell.