[QE-users] Time constant of Nose-Hoover chain

[Leonid Kahle]

Hello Sergio,

the time constant of the thermostat is a parameter of your calculation 
named fnosep in the namelist IONS 
(https://www.quantum-espresso.org/Doc/INPUT_CP.html#idm306).
Therefore, it's either what you set it to or the default (1THz), and 
printed in the output as:  nose` frequency(es)       = ....

"how could I infer if thermostat is interacting strong or weakly 
according to the magnitude of the time constant":
So, the parameter fnosep is a frequency. To infer how strongly a 
thermostat of that frequency couples with your system, you need to 
calculate the vibrational density of states (VDOS, also called power 
spectrum) of your system, which is the fourier transform of the 
velocity-velocity autocorrelation function ideally from an MD trajectory 
without a thermostat (NVE).

The thermostat, generally speaking, will interact strongly with modes of 
the same frequency as the thermostat (said fnosep) and weakly with modes 
of different frequencies. (see 6.1 of Frenkel & Smit: Understanding 
Molecular Simulation: From Algorithms to Applications).

Best,

     Leonid

PS: To calculate the power spectrum, there is a Python package to do it, 
and you can check the code here: 
https://github.com/lekah/samos/blob/master/samos/analysis/dynamics.py, 
function get_power_spectrum


On 1/27/20 5:25 AM, Sergio Castaneda Ramirez wrote:
>
> Dear experts,
>
> I am carrying out a NVT simulation for a hydrated anion-exchange 
> membrane segment using cp.x. I am using Nose-Hoover chains as 
> thermostat and I need to show if the thermostat is interacting strong 
> or weakly with the system via the time constant of the thermostat.
>
> Therefore, I cordially want to ask you if the time constant of the 
> thermostat is directly reported in one of the output files of cp.x or, 
> if not reported, how could I estimate it from output information? 
> Also, I want to ask you, how could I infer if thermostat is 
> interacting strong or weakly according to the magnitude of the time 
> constant?
>
> Thanks a lot in advance,
>
> Sergio
>
>
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