The second-order reduced density matrix method and the two-dimensional Hubbard model

► Accurate ground state energies of 4 × 4 two-dimensional Hubbard model were obtained. ► By using the second-order reduced density matrix. ► Employing N-representability as P, Q, G, T1 and T2′. ► Highly correlated region of parameter 4 < U/t < 10. ► The energy deviations per site were only ∼0.01.