I have been using the ASE-SIESTA-PyNAO module to attempt some RAMAN/IR mode calculations on graphene (if anyone has any experience with this please send me a message maybe we can chat about it). At no point during these calculations do I define mass of carbon (neither in the PyNAO arguments nor in SIESTA species definition arguments which is very strange of course !) to get the RAMAN/IR spectra and I get shifted results (both blue and red shifts, which seem somewhat random) with respect to what a typical RAMAN spectrum of graphene would look like.
I have also used the SIESTA/VIBRA module to extract all modes at the Gamma point, those do require mass, which makes sense, but I can't seem to find a method to extract the modes that are RAMAN or IR active from the total modes I obtain from VIBRA.
My question here is thus in three parts:
Can anyone explain how ASE calculates IR or RAMAN modes without implicating mass of the atoms ? (and if it does where can I toggle it to get better results).
Does anyone have a method for extracting RAMAM or IR active modes from the total modes we can obtain with the VIBRA module from SIESTA ?
Since the method from ASE-PyNAO-SIESTA differs completely from the SIESTA/VIBRA method to calculate vibrational modes, it seems impossible to parse VIBRA results through the ASE calculator to get RAMAN/IR activity. Is this assumption correct ?