[DFTB-Plus-User] Query on the use of DFTB+ for solid state calculations

[Benjamin Hourahine]

Hello Bhuvaneswari R,

You may also need to test for convergence with respect to numerical parameters of the calculation.

A molecular crystal will probably converge for gamma-point-only calculations but may not. The default SCC tolerance is probably accurate, but depending on the unit cell, the geometry optimizer might not converge to less than 2% geometric changes if the cell has an unusual shape.

Regards

Ben

Dr. B. Hourahine
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________________________________
From: DFTB-Plus-User <dftb-plus-user-bounces at mailman.zfn.uni-bremen.de> on behalf of Bálint Aradi <aradi at uni-bremen.de>
Sent: 08 March 2023 08:23
To: dftb-plus-user at mailman.zfn.uni-bremen.de <dftb-plus-user at mailman.zfn.uni-bremen.de>
Subject: Re: [DFTB-Plus-User] Query on the use of DFTB+ for solid state calculations

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Dear Bhuvaneswari R,

yes, you use the right parameter set. You should take into account, that
minimal basis methods (DFTB, xTB) usually use strongly compressed atomic
orbitals as basis in order to describe intramolecular chemical bonds
correctly. As a consequence, the intermolecular interactions might be
less precise, as the basis functions decay too fast outside of the
molecule. Therefore, depending on your system, the 2% limit might be too
high.

Best regards,

Bálint



On 01.03.23 16:12, Bhuvaneswari Ramaganesan wrote:
> Greetings,
>
> This is Bhuvaneswari R, Ph. D student at JNCASR, India. I am currently
> using DFTB+ to optimize a molecule (hexachlorobenzene) so that I can
> create a molecular crystal out of it. In order to optimize the molecule,
> I used the default dispersion parameters of DFTD3 and Slater Koster
> files (3ob) from DFTB. The error in lattice constants after optimization
> is greater than 2 %.
>
> Am I using the right Slater-Koster files? Is there any modified file for
> solid-state calculations? To achieve accurate optimization (within error
> bar limit), how can the dispersion parameters be tuned? Can you please
> help in choosing the right parameters? Thank you.
>
> Regards,
> Bhuvaneswari R
>
>
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Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
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