Quantum mechanical calculations of infrared spectra return the absorption coefficient times the refractive index, which is effectively the dielectric function. But this result is compared with absorbance spectra. The higher the oscillator strength, the greater become the differences (see, e.g. Article Dispersion related Coupling Effects in IR spectra on the exa...
). Accordingly, e.g., for the water HO stretching vibrations, it makes no sense to alter the calculations to better fit the absorbance bandshape, because those of absorbance bands and the dielectric function are different. Why is this not taken into account?
The dielectric function is a dependence of the complex relative permittivity (dielectric constant) on radiation frequency. Proceeding the complex dielectric constant one could calculate absorption and refraction.
Igor L Glukhov Yes, I fully agree - either one processes the result of the quantum mechanical calculations further (actually, the outcome is similar to the imaginary part of the dielectric function) or the experimental spectra to the point where apples are compared with pears...
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