[CP2K-user] [CP2K:19712] Temperature during NVE run

[Léon Luntadila Lufungula]

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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