Approximate ground state of the Lipkin model hamiltonian in density-matrix formalism
A method for finding the ground state of a many-body system, which is consistent with the truncation scheme used in the time-dependent density-matrix formalism of Wang and Cassing, is presented and applied to the Lipkin model. It is found that the method gives better ground states than the Hartree-Fock approximation. However, they significantly differ from the exact ground states.