[gmx-users] Vibrational spectrum from normal mode analysis - regd

[ramesh cheerla]

Dear all,

    I have done normal mode analysis (NMA) for a polymer crystal using all
atom force field and generated the vibrational spectrum (using the command
g_nmeig_d  -f  hessian.mtx    -s  topol.tpr   -os  spectrum.xvg   ). But
the spectrum that I have generated  by using above command from NMA  is not
matching with the one generated from velocity auto correlation function
(i.e., g_velacc),  especially in low frequency region(high frequency
regions are matching). Moreover the spectrum obtained from g_velacc seems
to be matching with the existing literature.
Here my doubts are i) why these two spectra are not identical ?
ii) Am I  making anything wrong in generating  NMA spectrum ?

Can anybody please help me in this regard.

Thank you in advance,

Best,
Ramesh