Angular-momentum projection in coupled-cluster theory: Structure of Mg-34

Single- reference coupled-cluster theory is an accurate and affordable computational method for the nuclear many-body problem. For open-shell nuclei, the reference state typically breaks rotational invariance and angular momentum must be restored as a good quantum number. We perform angular-momentum projection after variation and employ the disentangled coupled-cluster formalism and a Hermitian approach. We compare our results with benchmarks for 8Be and 20Ne using a two-nucleon interaction from chiral effective field theory and for pf -shell nuclei within the traditional shell model. We compute the rotational band in the exotic nucleus 34Mg and find agreement with data.