[CP2K-user] [CP2K:19715] Re: Temperature during NVE run

[Léon Luntadila Lufungula]

Hi Matt,

Thanks for the informative answer! I'm a real novice in MD simulations so 
that obvious explanation didn't occur to me..

Kind regards,
Léon

On Tuesday 9 January 2024 at 12:12:39 UTC+1 Matt Watkins wrote:

> Hi, yes, if you start from a more or less optimised structure you expect 
> around half of the initial kinetic energy to be converted into other modes 
> (potential energy has to rise from the minimum so kinetic energy will 
> decrease).
> To just get a ball park temperature you can use the temperature rescaling 
> algorithm to just rescale the kinetic energy if it drops too low.
> Matt
>
> On Tuesday 9 January 2024 at 10:59:24 UTC Léon Luntadila Lufungula wrote:
>
>> Dear all,
>>
>> I'm running some ab initio NVE calculations to determine accurate 
>> settings for EPS_SCF, EPS_DEFAULT, CUTOFF and TIMESTEP for my production 
>> runs. I think I have found good values for each as I see little to no drift 
>> in the energy after 16ps of simulating and the oscillations also seem 
>> reasonable (see below for details). However, I see that the average 
>> temperature during my simulation is around 157 K, while I set TEMPERATURE 
>> to 298.15 K. (see input and ener files in attachment)
>> [image: T.png]
>> I know that NVE does not keep the temperature constant, but is it normal 
>> that it is nearly half of the set temperature? Any comments on this would 
>> be greatly appreciated!
>>
>> MD convergence details (comments on this part are always welcome):
>> The extrapolated drift per ns is: 0.0001415478264154313 a.u 44.697268403725126 
>> K The oscillations (standard deviation) of the energy is: 2.6854768188532883e-05 
>> a.u. The average kinetic energy is: 0.06610813204255318 a.u. The average 
>> CPU time is: 8.164207377190335 The drift per ps is 0.0002 % of the 
>> kinetic energy. The oscillations are 0.0406 % of the kinetic energy.
>> [image: E.png][image: U_and_E.png]
>> The time until equilibration was chosen to be 5ps based on the figures 
>> above and all mentioned quantities above have been calculated from 5ps to 
>> the final step of the simulation (~16ps).
>> Drift was calculated by taking the difference in the cumulative average 
>> of the constant of motion over the equilibrium period (5-16ps) and dividing 
>> by the timerange (~11ps) to get the drift per ps. This was then multiplied 
>> by 1000 to get the drift per ns.
>> The oscillations were calculated by taking the standard deviation of the 
>> constant of motion over the equilibrium period.
>> As instructed by the manual 
>> <https://manual.cp2k.org/trunk/methods/sampling/molecular_dynamics.html> in 
>> the section of MD Convergence, the drift per ps and oscillations are 
>> calculated as percentages of the kinetic energy. As these values are well 
>> below 1% and the constant of motion seems to be quite stable in the 
>> pictures, I can conclude that the settings are appropriate to maintain 
>> stable dynamics. EPS_SCF 6 gave a significant drift, therefore EPS_SCF 7 
>> was chosen. I'm also looking into lower values for EPS_DEFAULT to reduce 
>> the CPU time.
>>
>> If someone has comments or suggestions on my way of calculating and 
>> interpreting the drift and oscillations, I'd be happy to hear them!
>>
>> Best wishes for 2024!
>>
>> Kind regards,
>> Léon
>>
>>

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