  Phonon-phonon interaction is systematically studied by nonequilibrium Green's
function (NEGF) formulism in momentum space at finite temperatures. Within the
quasi-particle approximation, phonon frequency shift and lifetime are obtained
from the retarded self-energy. The lowest order NEGF provides the same phonon
lifetime as Fermi's golden rule. Thermal conductance is predicted by the
Landauer formula with a phenomenological transmission function. The main
advantage of our method is that it covers both ballistic and diffusive limits
and thermal conductance of different system sizes can be easily obtained once
the mode-dependent phonon mean free path is calculated by NEGF. As an
illustration, the method is applied to two one-dimensional atom chain models
(the FPU-\beta model and the \phi^4 model) with an additional harmonic on-site
potential. The obtained thermal conductance is compared with that from a
quasi-classical molecular dynamics method. The harmonic on-site potential is
shown to remove the divergence of thermal conductivity in the FPU-\beta model.
