#### fluidistic

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This leaves me entirely knowledgless about how to compute the emissivity from scratch. I do not even know whether it is a purely electronic phenomenon for metals, and/or an atomic/ionic phenomenon for semiconductors and insulators.

I have found the paper https://www.researchgate.net/profile/Stanislav_Avdoshenko/publication/265855757_High-Temperature_Emissivity_of_Silica_Zirconia_and_Samaria_From_ab_Initio_Simulations_Role_of_Defects_and_Disorder/links/561d25b508aecade1acb3409.pdf which suggests that the emissivity can be found by calculating the dielectric constant, which makes entirely sense to me because it is related to the EM absorption of the material. But then it is said that such a quantity can be calculated using an ab initio approach. I do not know whether this is valid for any material, specially non-metallic ones. What about materials without free electrons? How would one approach the problem?