How optical absorption spectra are calculated on the Materials Project (MP) website.
The optical absorption spectra is obtained by calculating the frequency-dependent dielectric tensors using VASP. It uses independent particle approximation and assumes only vertical interband transitions to obtain the imaginary part of the dielectric tensors. Via the Kramers-Kronig relations the relationship between the dispersions of the real and imaginary parts of the dielectric function can be established. With both the real and imaginary part of the frequency-dependent dielectric tensors, one can calculate the optical absorption coefficient at different photon energies. Our results are validated against the experimental database: https://refractiveindex.info