[CP2K-user] [CP2K:16441] Power Spectrum.Help! not zero at 0 cm-1

[Lucas Lodeiro]

Hi,

The non zero value at zero frequency is typical when you have a
system/molecule with translational and rotational movement.
It is not clear if you are simulating a single molecule (without PBC) or a
bulk system (with PBC). For the former case, you could control both with
COMVEL_TOL, ANGVEL_TOL and ANGVEL_ZERO keywords. For the second, the
angular movement cannot be controlled and just the translation of COM
contributes to the zero frequency value. Also, the "natural" band width of
the FFT could affect it when a signal is near zero frequency and its tail
reaches the zero frequency.

About the MD... a timestep of 10 fs is a very huge one... if you have
hydrogens, 0.5 fs is the best, and maybe 1.0 fs could work. Also if you are
working with a bulk system, 14 ps of AIMD probably is not sufficient to
obtain an equilibrated system in term of structure and in terms of thermal
energy.

Regards - Lucas Lodeiro

El vie, 7 ene 2022 a las 0:58, Kai (<liangziwuli554 at gmail.com>) escribió:

> Hello everyone,
> I am trying to calculate the vibrational spectrum of an inorganic cluster
> using TRAVIS. It's simulated for 14 ps with a CSVR timeconstant of 1 ps and
> 10 fs timestep. I think I get the great shape of the  vibrational curve. *But
> it shows a non zero value at 0 cm-1.*This should be unreasonable. What
> should I do?
> I guess I should remove the 6 macroscopic degrees of freedom (3x
> translation, 3x rotation), but how?
>
> Please help me.
> Thanks all.[image: power spectrum.png]
>
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