Symmetry projection to coupled-cluster singles plus doubles wave function through the Monte Carlo method

A method for calculating the symmetry-projected energy of coupled-cluster singles plus doubles (CCSD) wave function through the Monte Carlo method is proposed. We present benchmark calculations in considering the three-level Lipkin model which is a simple and minimal model with two phases: spherical and deformed. It is demonstrated that this method gives good ground-state energy and low-lying spectra.