Shell-model coupled-cluster method for open-shell nuclei

We present an approach to derive effective shell-model interactions from microscopic nuclear forces. The similarity transformed coupled-cluster Hamiltonian decouples the single-reference state of a closed-shell nucleus and provides us with a core for the shell model. We use a second similarity transformation to decouple a shell-model space from the excluded space. We show that the three-body terms induced by both similarity transformations are crucial for an accurate computation of ground and excited states. As a proof of principle we use a nucleon-nucleon interaction from chiral effective field theory, employ a ^4He core, and compute low-lying states of ^6--8He and ^6--8Li in p-shell model spaces. Our results agree with benchmarks from full configuration interaction.