We merge two successful ab initio nuclear-structure methods, the no-core
shell model (NCSM) and the multi-reference in-medium similarity renormalization
group (IM-SRG) to define a new many-body approach for the comprehensive
description of ground and excited states of closed and open-shell nuclei.
Building on the key advantages of the two methods---the decoupling of
excitations at the many-body level in the IM-SRG and the access to arbitrary
nuclei, eigenstates, and observables in the NCSM---their combination enables
fully converged no-core calculations for an unprecedented range of nuclei and
observables at moderate computational cost. We present applications in the
carbon and oxygen isotopic chains, where conventional NCSM calculations are
still feasible and provide an important benchmark. The efficiency and rapid
convergence of the new approach make it ideally suited for ab initio studies of
the complete spectroscopy of nuclei up into the medium-mass regime.Comment: 5 pages, 4 figures, v2: update to published versio