[CP2K-user] [CP2K:19843] Cons. Qty for different ensembles + Pressure fluctuation

[Krack Matthias]

Dear Léon

MD based on DFT compared to classical potentials suffers more from numerical noise in the forces. Therefore, you should reduce EPS_DEFAULT from 1.0E-10 to at least 1.0E-12 in order to reduce that noise. Nevertheless, you will always observe a noticeable energy drift when you run long enough.

HTH

Matthias

From: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Léon Luntadila Lufungula <Leon.luntadilalufungula at uantwerpen.be>
Date: Monday, 29 January 2024 at 11:03
To: cp2k <cp2k at googlegroups.com>
Subject: Re: [CP2K:19840] Cons. Qty for different ensembles + Pressure fluctuation
Dear Matthias,

My apologies, I forgot to add the input files. You can find them in attachment to this message.

Kind regards,
Léon
On Monday 29 January 2024 at 10:36:30 UTC+1 Léon Luntadila Lufungula wrote:
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...
[https://groups.google.com/group/cp2k/attach/9fcbf3dc22a3/H2O-DFT-6x6x6-equilibration.png?part=0.2&view=1]
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?
[https://groups.google.com/group/cp2k/attach/9fcbf3dc22a3/a101-solv10-NPT_F-equilibration.png?part=0.1&view=1]

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.

[https://groups.google.com/group/cp2k/attach/7718930ba853e/H2O-TIP3P_FLEXIBLE-6x6x6.mp-NPT-plots.png?part=0.1&view=1]

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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