High-frequncy oscillations in a NPT ensemble

[Ider]

Dear cp2k developers and users,

I extended the test simulation with the cutoff of 1200Ry for 1.7ps. I 
attached the input and restart files together with the output files ( 
genereal output, enegry, cell and stress files), The oscillations still 
remain. 

I will be very grateful for your help!

Best regards,

Ider

Am Dienstag, 25. November 2014 14:47:43 UTC+1 schrieb Ider:
>
>
> Dear cp2k developers and users,
>
> I would like to ask for some help with some issues I am having with AIMD 
> simulation in the  NPT ensebmle.  I attached the input file "npt.inp" as 
> well as "stress"- and "cell"-files. For convenience I also added plots of 
> the cell along c and zz component of the stress tensor.  
> My goal is to equilibrate a model of a Ge2Sb2Te5 (GST225) crystal phase 
> (108 atoms) at T=600K and p=0 kbar. What I did was to relax the coordinates 
> and the cell parameters and then run an equilibration of the system at 600K 
> at constant volume ( ensemble=langevin ) which works fine. Then I switched 
> the ensemble to NPT_I using the canonical velocity rescaling thermostat 
> (Bussi thermostat) and the barostat with the default settings ( I tried 
> both TIMECON=100 fs / TIMECON = 1000fs ). However I observe a strange 
> high-frequency oscillations ( time period = 20-30 fs !! ) of the stress and 
> the cell parameters, which looks very systematic. This oscillations remains 
> even for longer simulation times ( see the plot "long-cell-c.png" , in 
> which it was slightly different setting with flexible cell / without vdW). 
>
> I tested the effect of following parameters on the behaviour of the 
> system. Well the simulation time testing was quite short (varying 100-1000 
> steps). No significant change of the oscillation on this simulation time 
> scale could be observed.
>
> 1. CUTOFF= 300 / 600 / 1200Ry.      
>  
> 2. Larger reference cell so that the grid density is constant. In the 
> input file "npt.inp" this section is uncommented, however in the restart 
> file it is present. 
>     &CELL_REF
>        ABC 13.5 15.0 20.0
>     &END CELL_REF
>
> 3. TIMECON = 100 / 1000 / 2000 / 10000 fs. 
> 4. TEMPERATURE of the BAROSTAT = 60K. 
> 5. I also tried some options in the XC_GRID section. For example RHO = 
> SPLINE3 or XC_DERIV = SPLINE3 / NN50 /NN50_SMOOTH. No significant 
> improvements. 
>
> The stacking of the layers in this GST225 model is in the "inverted 
> Petrov" sequence where Ge and Te are swicthed. I also tested the stacking 
> in the "Petrov" sequence, but similar oscillations are present. 
>
> I would be truly grateful for any advice!
>
> Best regards,
>
> Ider
>
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