In recent years, one of the most common applications has been the calculation of mode lifetimes for predictions of lattice thermal conductivity. To a first approximation, thermal conductivity from finite lifetimes due to phononphonon scattering can be calculated from perturbation theory on top of the harmonic phonon dispersion, typically based on third-order force constants. This perturbative treatment, however, does not result in a correction to imaginary modes or modification of the free energy.