Temperature oscillations in QM/MM MD

Dawid das add... at googlemail.com
Sat Jun 9 12:20:27 UTC 2018

Hello Matt,

Thank you for your answer. And what if the temperature for QM subsystem is 
smaller than temperature 
of whole system? For instance ca. 80 K and 100 K, respectively.
Is it a sign of bad thermostating or can I expect it and there is not much 
one can do about it?

Best wishes,
Dawid Grabarek

W dniu piątek, 8 czerwca 2018 20:30:13 UTC+2 użytkownik Matt W napisał:
> 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,
> Matt
> 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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