[CP2K-user] [CP2K:19801] Cons. Qty for different ensembles + Pressure fluctuation

[Krack Matthias]

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: cp2k at googlegroups.com <cp2k at googlegroups.com> on behalf of Léon Luntadila Lufungula <Leon.luntadilalufungula at uantwerpen.be>
Date: Tuesday, 23 January 2024 at 11:47
To: cp2k <cp2k 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.

[cid:dac9197f-4388-4dcc-bd2a-ecdcaa1651a3]
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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