[CP2K-user] [CP2K:11087] DFTB simulations of liquid water

[Thomas Kühne]

Dear Guillaume, 

I have a couple of (hopefully helpful) comments using DFTB within CP2K: 
1. To the best of my knowledge only the mio and znorg parameter sets, 
i.e. the ob2 parameter set needs to be downloaded from dftb.org <http://dftb.org/>
2. IMHO, the usage of a damped gamma does not represent a general 
improvement, it marginally improves the 1st peak for water but appears 
to be worse for a lot of other systems
3. I rather suggest to use the improved parametrization of Dömer et al., 
see e.g. https://arxiv.org/pdf/1408.5161.pdf <https://arxiv.org/pdf/1408.5161.pdf> and references therein. 
Since I couldn’t find the files anywhere else, I have attached them to the 
present email. We will check them again and upload them to the repository 
next week. 
4. For comparison have a look at Fig. 1-3 in https://arxiv.org/pdf/1512.08206.pdf <https://arxiv.org/pdf/1512.08206.pdf> 
for a plain DFTB calculation of liquid water by CP2K with the provided original 
parametrization, without the modified gamma and no vdw correction, but including 
nuclear quantum effects. No inhomogeneity in the particle density have been observed. 
5. Please also have a look at the recent discussion about the SPME settings, 
which may play a role in your observed behavior.

Best, 
Thomas



> Am 26.12.2018 um 16:33 schrieb Guillaume Stirnemann <gsti... at gmail.com>:
> 
> Hi all, 
> 
> I am trying to simulate a box of liquid water using DFTB with periodic boundary conditions. My initial configuration is taken from a previous classical MD run performed in the NVT ensemble at the experimental density. 
> I have tried different set-ups, including SCC-DFTB ("DFTB2") and DFTB3-diag, both with or without Grimme's D3 corrections, all ran in the NVT ensemble for 50 ps. 
> In all cases, the simulations run well, but I end-up with a large hole in the simulation box. When comparing my radial distribution functions that of Cui who used DFBT+ (JPCB 115, 6790 (2011)), the first peak of the O-H rdf is usually a bit more pronounced but the contraction of the whole system is more visible on the second hydration shell. The differences are less visible for the O-O rdfs. The hole occupies about 1/2 of the simulation box, so the final density of my liquid phase is about 2! 
> 
> I attach the initial configuration, and the cp2k input file for DFTB2 with no D3. I tried on two different machines, one with 6.1 and the other one with 5.1. 
> 
> I am not an expert of CP2K so there might be some issues with my input, and your help would be very much appreciated. 
> 
> Best wishes
> 
> Guillaume 
> CNRS Institut de Biologie Physico-Chimique, Paris, France
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> <input-DFTB2-gamma-noD.in><h2o.xyz>

==============================
Prof. Dr. Thomas D. Kühne
Dynamics of Condensed Matter
Chair of Theoretical Chemistry
University of Paderborn
Warburger Str. 100
D-33098 Paderborn
Germany
tdku... at mail.upb.de
+49/(0)5251/60-5726

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