[DFTB-Plus-User] Optimizing polyyne chain in DFTB+ for low BLA range: Need guidance

[Benjamin Hourahine]

Hello Khadidja,

BLA is fairly sensitive to functional in DFT, with mean-field functionals (LDA/GGA) often getting this incorrect. DFTB2 or 3 has the same issues, so as with DFT, either hybrid functionals or +U corrections may be needed.

The BLA for polyacetylene is definitely closer to the experimental values with these corrections. Hybrids should be used with ob2, while +U can be applied on top of the mio set.

Regards

Ben

Dr. B. Hourahin
Senior Lecturer and Director of Student Support
Department of Physics, SUPA
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+44 141 548 2325, benjamin.hourahine at strath.ac.uk

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________________________________
From: DFTB-Plus-User <dftb-plus-user-bounces at mailman.zfn.uni-bremen.de> on behalf of Khadidja Chaib Edderraa <k.chaibedderraa at univ-chlef.dz>
Sent: 02 June 2023 13:29
To: dftb-plus-user at mailman.zfn.uni-bremen.de <dftb-plus-user at mailman.zfn.uni-bremen.de>
Subject: Re: [DFTB-Plus-User] Optimizing polyyne chain in DFTB+ for low BLA range: Need guidance

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Hello,
I am writing to you to provide further details regarding the structure I used before optimization for the chain. The structural parameters I employed were d1=1.51 Å and d2=1.22 Å. However, after performing the relaxation, I obtained a BLA value of approximately 0.11 Å, which is significantly larger than the range predicted by DFT (0.01-0.05 Å).

On Fri, Jun 2, 2023, 12:10 Khadidja Chaib Edderraa <k.chaibedderraa at univ-chlef.dz<mailto:k.chaibedderraa at univ-chlef.dz>> wrote:
Dear ,

I hope this email finds you well. I am currently working on optimizing a polyyne chain using DFTB+ and aiming to achieve a low range of bond length alternation (BLA) in my system. After exploring the available resources, I believe your expertise in DFTB+ could provide valuable insights and guidance for my research.

To provide some context, my research focuses on  ensuring that the molecular structure represents the most stable configuration and optimizing the polyyne chain is crucial for achieving BLA values  that align closely with experimental and other simulation data.

The specific details of the polyyne chain I'm studying are as follows:
- Number of carbon atoms: 2 carbon
- Parameterisation data (Slater-Koster files)  used to perform calculations : 3ob, mio, ob2, matsi
- Geometry optimization:  using  conjugate gradient algorithm but have not obtained the desired results.

My main questions are as follows:
1. What are the recommended optimization settings or parameters for a polyyne chain in DFTB+ to achieve a low BLA range?
2. Are there any specific convergence criteria that I should consider during the optimization process?
3. Are there any DFTB+ features or functionalities that could be particularly useful for optimizing polyyne chains and reducing the BLA range?

Thank you very much, and I look forward to your response.

Sincerely,
Khadidja
k.chaibedderraa at univ-chlef.dz<mailto:k.chaibedderraa at univ-chlef.dz>
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