I am trying to understand the IR spectra of liquid. I can get the autocorrelation function of atoms' velocity,(adsbygoogle = window.adsbygoogle || []).push({});

<v_{i}(0)v_{i}(t)>

make a Fourier Transformation, the vibrational density of state (VDOS) can be obtained. Does the VDOS always be positive? Or it can also take negative value in some frequency region? Thanks

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# Is the Fourier transf. of an autocorrelation functn always positive?

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