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3:16:08 PM PDT  Tue, Aug 23rd 2016 

Hi Aniruddha,
Is your classical MD simulation in the condensed phase ? If so, you should be able to calculate the vibrational spectra with the information you have from the MD simulation. Basically, you calculate the autocorrelation function of the velocities. This will give you the vibrational density of states. This will include both IR and Raman modes. To calculate the IR specifically, you need to calculate the autocorrelation function of the molecular dipole.
If you have a molecular system and if you want to calculate the IR at the DFT level of theory, then define the geometry in the geometry block and ask for task dft frequencies. See the following link for more information
http://www.nwchemsw.org/index.php/Release66:Hessians_%26_Vibrational_Frequencies
Hope this helps.
Best,
Niri
niri.govind@pnnl.gov



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