Active space coupled cluster calculation of open-shell nuclei

We present an active space coupled-cluster approach to the calculations of open-shell nuclei. The Hilbert space of the Hamiltonian is split into an active and external space. Cluster operators in the active space are determined through a spherical shell model calculation. The contributions of 4-particle-4-hole excitations adjacent to the Fermi surface are captured, and the rotation invariance of the system is preserved in the active space. The cluster operators in the external space (coupled to the active space) are determined through the standard coupled cluster. We applied the method to the low-lying states of ²⁰Ne and ¹²C and compared it with the angular momentum projected coupled-cluster calculations.