Ab initio coupled-cluster theory for open-shell nuclei
Abstract
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 2 nucleons outside a closed shell. We perform proof-of-principle calculations for the ground states of the helium isotopes 3-6He and the first excited 2+ state in 6He. 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 4He play an important role for the accurate description of 6He. For the open-shell nucleus 6He, 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.