[CP2K-user] [CP2K:19838] Cons. Qty for different ensembles + Pressure fluctuation

[Léon Luntadila Lufungula]

Dear Matthias,

I've also tried to equilibrate a box of 216 water molecules with DFT and 
while the temperature and pressure seem to equilibrate quite easily, the 
constant of motion shows large jumps despite the potential and kinetic 
energy also being quite constant... If I'm not mistaken, the constant of 
motion in the NPT ensemble is the energy of the system + the energy of the 
thermostat + energy of barostat, as shown in this lecture 
<https://www.cp2k.org/_media/events:2016_summer_school:cp2k_moving_atoms.pdf> 
from 2016, so this would suggest that the fluctuations are due to the 
thermostat or barostat, right? Like the TIP3P NPT run, I used a thermostat 
timecon of 100 fs and a barostat timecon of 1000 fs and after 21 125 fs 
(blue vertical line in figures), I restarted the calculation and increased 
the thermostat timecon to 500 fs. I accidentaly set RESTART_BAROSTAT and 
RESTART_BAROSTAT_THERMOSTAT to FALSE upon restarting, but I'm unsure if 
this might have lead to the large jumps in the constant of motion later at 
t > 28 000 fs... 
[image: H2O-DFT-6x6x6-equilibration.png]
Same goes for the NPT_F calculation of my solvated TiO2 slab. I did a 10 ps 
simulation with thermostat timecon 50 and barostat timecon 100 and 
everything seems to equilibrate except for the constant of motion which 
seems to steadily rise. After 10 ps of simulation I restarted the 
calculation and again accidentaly messed up my input and set 
RESTART_BAROSTAT and RESTART_BAROSTAT_THERMOSTAT to FALSE but this time I 
didn't even change the timecons of the thermostat and barostat, but this 
time I immediately see a significant jump in the constant of motion after 
which it continues to increase... Any idea how I can fix this so that the 
constant of motion stays...well...constant?
[image: a101-solv10-NPT_F-equilibration.png]

Kind regards,
Léon

On Tuesday 23 January 2024 at 14:41:46 UTC+1 Léon Luntadila Lufungula wrote:

> Dear Matthias,
>
> My AIMD calculations are on a slab wherein I keep my bottom layer fixed, 
> but apparently REGION MASSIVE is not compatible with constraints, so I'll 
> have to use REGION GLOBAL. Either way, thanks a lot for the detailed 
> answer! For a novice MD practicioner as myself this is already enough 
> information to get started! I'll try the values you suggested during my 
> production run while keeping an eye on the temperature and pressure during 
> the simulation; I guess when I've chosen my TIMECON too large, this will 
> result in the system drifting away from the equilibrium values.
>
> As always, thanks a lot for all the help!
>
> Kind regards,
> Léon
>
> On Tuesday 23 January 2024 at 14:21:05 UTC+1 Krack Matthias wrote:
>
>> Dear Léon
>>
>>  
>>
>> Using a MASSIVE CSVR thermostat with a small TIMECON of about 50 is 
>> appropriate and efficient for equilibration. Thereafter, the TIMECON for 
>> the thermostat can be increased to 500-1000 for the sampling (production) 
>> runs and also a GLOBAL instead of a MASSIVE thermostat can be used, if 
>> possible, to reduce the impact of the thermostat on the dynamics. You will 
>> find different opinions about the absolute and relative values for the 
>> TIMECONs of thermostat and barostat in the web or literature. I suggest to 
>> use a two to five times larger TIMECON for the barostat compared to the 
>> thermostat. You have, however, to keep in mind that the appropriate (best) 
>> parameters for baro- and thermostat depend on the type of system and the 
>> selected conditions eventually.
>>
>>  
>>
>> HTH
>>
>>  
>>
>> Matthias  
>>
>>  
>>
>> *From: *cp... at googlegroups.com <cp... at googlegroups.com> on behalf of 
>> Léon Luntadila Lufungula <Leon.luntad... at uantwerpen.be>
>> *Date: *Tuesday, 23 January 2024 at 11:47
>> *To: *cp2k <cp... at googlegroups.com>
>> *Subject: *Re: [CP2K:19801] Cons. Qty for different ensembles + Pressure 
>> fluctuation
>>
>> Hi Matthias,
>>
>>  
>>
>> So I started an NpT directly from the inputs you gave me with a box 
>> containing 216 water molecules. After a 300ps equilibration (plots thus 
>> starrt at 0.3 ns), the pressure fluctuations are again in the 10kbar range 
>> which is okay as you pointed out, while the average pressure is around -14 
>> bar, so tha'ts also within the 20 bar range you mentioned. The energy drift 
>> per atom is also below 1K so everything seems to work reasonably. 
>>
>>  
>>
>>
>> I'm starting my AIMD simulations as we speak, so hopefully all goes well 
>> there too. I read that it is best to do a short equilibration with a strong 
>> coupling to the thermostat (e.g. CSVR, TIMECON 20 and REGION MASSIVE), 
>> before going to a weaker coupling (e.g. CSVR, TIMECON 200 and REGION 
>> GLOBAL), but is there also a rule of thumb w.r.t. the barostat TIMECON? I 
>> regularly see TIMECON 1000, but is it useful to set it to TIMECON 100 in 
>> this early equilibration?
>>
>>  
>>
>> Thanks for all the help already, it really helps me a lot given the fact 
>> that I don't have a senior researcher with a lot of hands-on CP2K 
>> experience around!
>>
>>  
>>
>> Kind regards,
>>
>> Léon
>>
>>  
>>
>

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