[DFTB-Plus-User] DFTB+ incorrect density in MD for water

[Luca Babetto]

Dear Bálint,

Thank you kindly for your reply.

I relaunched the calculations with the 3rd order corrections as well as DFT-D3 dispersion corrections as you suggested.

I already tested briefly the xTB Hamiltonian and it did seem to behave better in this regard, but for our workstation it is MUCH slower than DFTB for a system of this size, so we cannot really use it yet for AIMD with our resources.

On this subject, I noticed that the dftb.org website mentions the "DampXHExponent" variable in the 3ob parameter page, which I assume is the HCorrection/Damping option you are talking about, as the exponent is the same (4.00). Is this correct? Could it be worth updating either the manual or more likely the website to avoid confusion? I could not find the "DampXHExponent" keyword in the 22.1 manual, so I assume that keyword has been superseded by HCorrection.

Thank you again for the help, I'll let you know how this simulation goes!

Kind regards

Luca


________________________________

Dear Luca,

sorry, indeed, your previous post apparently went done between other
threads. In that case, it is OK to re-post, for sure. And your post was
definitely as specific as it can be. :-)

I am not an MD and water expert, but here a few comments, which may help
you, nevertheless.

* When using the 3ob set, make sure to use HCorrection / Damping and
ThirdOrderFull options.

* The Berendsen-barostat we have implemented is known for fluctuating a
lot, but I have no clue, how severe it would be with water and which
time scale to look for (maybe somebody more expert in MD can comment on
that).

* I think, dispersion may indeed be important.

* Last, not least, the water description with the DFTB Hamiltonian is
not really wonderful, mostly due to the missing multipoles in the model.
So, it may sense, as done in

https://pubs.acs.org/doi/full/10.1021/jp1010555

to get the water box initial geometry from somewhere else, and do NVT
afterwards.

* Depending, what you are after, you could also test the xTB Hamiltonian
in DFTB+.

Best regards,



Bálint
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