  We introduce a hybrid many-body approach that combines the flexibility of the
No-Core Shell Model (NCSM) with the efficiency of Multi-Configurational
Perturbation Theory (MCPT) to compute ground- and excited-state energies in
arbitrary open-shell nuclei in large model spaces. The NCSM in small model
spaces is used to define a multi-determinantal reference state that contains
the most important multi-particle multi-hole correlations and a subsequent
second-order MCPT correction is used to capture additional correlation effects
from a large model space. We apply this new ab initio approach for the
calculation of ground-state and excitation energies of even and odd-mass
carbon, oxygen, and fluorine isotopes and compare to large-scale NCSM
calculations that are computationally much more expensive.
