Ionization structure in a pure hydrogen gas from four model codes using different approaches for computing the radiative transfer: C²-Ray and our method (ray-tracing), CRASH (Monte Carlo), and OTVET (variable Eddington tensor method). The model parameters are as specified by Iliev et al. (2006): particle density 10^-3 cm^-3; illumination by a monochromatic source emitting 5 × 10⁴⁸ hydrogen-ionizing photons/s; box size 6.6 kpc. Both ray tracing codes give practically identical results.