  In this paper, we extend the unified kinetic particle (UGKP) method to the
frequency-dependent radiative transfer equation with both absorption-emission
and scattering processes. The extended UGKP method could not only capture the
diffusion and free transport limit, but also provide a smooth transition in the
physical and frequency space in the regime between the above two limits. The
proposed scheme has the properties of asymptotic-preserving, regime-adaptive,
and entropy-preserving, which make it an accurate and efficient scheme in the
simulation of multiscale photon transport problems. The methodology of scheme
construction is a coupled evolution of macroscopic energy equation and the
microscopic radiant intensity equation, where the numerical flux in macroscopic
energy equation and the closure in microscopic radiant intensity equation are
constructed based on the integral solution. Both numerical dissipation and
computational complexity are well controlled especially in the optical thick
regime. A 2D multi-thread code on a general unstructured mesh has been
developed. Several numerical tests have been simulated to verify the numerical
scheme and code, covering a wide range of flow regimes. The numerical scheme
and code that we developed are highly demanded and widely applicable in the
high energy density engineering applications.
