Spherical coupled-cluster theory for open-shell nuclei

Background: A microscopic description of nuclei is important to understand the nuclear shell model from fundamental principles. This is difficult to achieve for more than the lightest nuclei without an effective approximation scheme. Purpose: Define and evaluate an approximation scheme that can be used to study nuclei that are described as two particles attached to a closed (sub-) shell nucleus. Methods: The equation-of-motion coupled-cluster formalism has been used to obtain ground- and excited-state energies. This method is based on the diagonalization of a non-Hermitian matrix obtained from a similarity transformation of the many-body nuclear Hamiltonian. A chiral interaction at the next-to-next-to-next-to leading order (N-3 LO) using a cutoff at 500 MeV was used. Results: The ground- state energies of Li-6 and He-6 were in good agreement with a no-core shell-model calculation using the same interaction. Several excited states were also produced with overall good agreement. Only the J(pi) = 3(+) excited state in Li-6 showed a sizable deviation. The ground- state energies of O-18, F-18, and Ne-18 were converged but underbound compared to experiment. Moreover, the calculated spectra were converged and comparable to both experiment and shell-model studies in this region. Some excited states in O-18 were high or missing in the spectrum. It was also shown that the wave function for both ground and excited states separates into an intrinsic part and a Gaussian for the center-of-mass coordinate. Spurious center-of-mass excitations are clearly identified. Conclusions: Results are converged with respect to the size of the model space and the method can be used to describe nuclear states with simple structure. Especially the ground-state energies were very close to what has been achieved by exact diagonalization. To obtain a closer match with experimental data, effects of three-nucleon forces, the scattering continuum, as well as additional configurations in the coupled-cluster approximations are necessary.