I want to calculate terahertz spectra for my system but I am confuse about how to do it. The program I am using (crystal17) has two options, one is to perform frequency calculations at gamma point and calculate IR spectra which by default comes from 0 to 4000 cm-1 . Another option is to do phonon dispersion calculations (http://tutorials.crystalsolutions.eu/tutorial.html?td=dispersion&tf=dispersion-ht) which generate supercell and perform calculations on it. I am confused about if the treahartz spectra will be the regular IR spectra taken between 0 to 100 cm-1 or it should be calculated from the phonon dispersion calculations.