Question about fix vacf

Dear Lammps users,

I’m writing to ask about fix vacf command. I understand that the velocity autocorrelation function might be used to capture diffusion coefficient. I saw that the fourier transform of velocity autocorrelation function results the vibration spectra and people use it ro compare with experimental IR spectroscopy data. However, my understanding is that IR spectra is resulted from the fourier transform of dipole moment ( i.e. Q.R or Q.V ) autocorrelation function which is different from the former one. So, does this comparison with experimental IR questionable or for example it is correct with certain simplification assumptions ? In other words, what is the phenomena that the Fourier transform of fix vacf results is implying at ?
Thanks in advance,

actually, you need the “transition moment”, which requires quantum calculations for proper spectra, since using the dipole moment does not distinguish between modes that are allowed or forbidden by symmetry and thus IR/Raman active or not.

whether you look at the fourier transform of velocity or dipole or position auto-correlation, the auto-correlation between each of those represents the frequency of fast or slow motions in any case. primarily the intensities are different. there are also different corrections for those.


Thank you Axel! I agree with your point on the “transition moment”. Regarding your second point, does it mean that there is no shift in the peak position / number upon doing the computation based on velocity or dipole ( Let’s say I’m only interested in the number and position of the peaks ) ?

within the accuracy of the classical model, yes. think about what the auto-correlation function represents: simplified, it is a time series of the “sameness” of a (vector) property and the recurrance of a certain degree of “sameness” at a constant interval will result in a peak in the fourier transform. now what is causing the “differentness” or “sameness” of the dipole vector?


Catch your point, thanks for the clarification.