Toward open-shell nuclei with coupled-cluster theory

We develop a method based on equation-of-motion coupled-cluster theory to describe properties of open-shell nuclei with A±2 nucleons outside a closed shell. We perform proof-of-principle calculations for the ground states of the helium isotopes He3-6 and the first excited 2⁺ state in He6. The comparison with exact results from matrix diagonalization in small model spaces demonstrates the accuracy of the coupled-cluster methods. Three-particle-one-hole excitations of He4 play an important role for the accurate description of He6. For the open-shell nucleus He6, the computational cost of the method is comparable with the coupled-cluster singles-and-doubles approximation while its accuracy is similar to the coupled-cluster with singles, doubles, and triples excitations.