Temperature oscillations in QM/MM MD

Matt W mattwa... at gmail.com
Fri Jun 8 18:30:12 UTC 2018

Hi again,

For such a small (sub)system there will be large oscillations - 140 atoms 
don't have a well defined temperature.  The oscillations should decrease as 
N^(-1/2), where N is number of atoms in the (sub)system.

Not that there might not also be problems in you simulation as well,


On Friday, June 8, 2018 at 1:23:18 PM UTC+1, Dawid das wrote:
> Dear CP2K Users,
> I am performing QM/MM MD simulation for system of almost 30 000 atoms of 
> which 140 create the 
> QM subsystem. The ensemble is NVT and thermostat CSVT with time constant 
> of 1 fs.
> My question is what are the expected fluctutations of temperature for the 
> QM part? The thermostat works
> nicely for total temperature but the QM temp. is either too low and too 
> high. I tried various time constants 
> and also region definitions and still I cannot meet the proper QM 
> temperature.
> Best regards,
> Dawid Grabarek
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