Measured diagonal of the covariance matrix for 7375 power spectra realisations of the density field using the Monte Carlo method (black line) up to k_N ∼ 0.67 h Mpc⁻¹. The other curves represent their predictions taking into account the Gaussian part alone (G) or by adding some non-Gaussian contributions of Eq. (15). For example in (1-NG) only the term in 𝒫³(k_i) is kept in the tri-spectrum development presented in Eq. (17), while in (3-NG) all of them are kept.