Ab initio coupled-cluster theory for open-shell nuclei

We develop a new method to describe properties of truly open-shell nuclei. This method is based on single-reference coupled-cluster theory and the equation-of-motion method with extensions to nuclei with $A\pm 2$ nucleons outside a closed shell. We perform proof-of-principle calculations for the ground states of the helium isotopes $^{3-6}$He and the first excited $2^+$ state in $^6$He. 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 $^4$He play an important role for the accurate description of $^6$He. For the open-shell nucleus $^6$He, the computational cost of the method is comparable with the coupled-cluster singles-and-doubles approximation while its accuracy is similar to coupled-cluster with singles, doubles and triples excitations.