... momentum[*]
This reference unnecessarily assumes an isotropic radiation stress. However, we note that the difference between the full stress-energy tensor (2) and the isotropic approximation has exactly the same form as the artificial viscosity tensor introduced in the same reference to numerically stabilize shock fronts. Hence, in all derivations in the above reference we may simply use the pressure \( \widetilde{p}=p+\rho J/3 \) for the isotropic part and set the viscosity coefficient, \( Q \), to \( \widetilde{Q}=-\rho \left( J/3-K\right) \), in order to obtain a description of radiation hydrodynamics that extends to the case where large radiation energies do not satisfy \( J\not \simeq 3K \). 
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...,[*]
Note that our definition of moments in Eq. (23) is twice as large as in standard references because this leads to more consistency between our code and its description. 
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