Re: [Phonopy-users] Reducing drift in force constants

[Greene, S. <sam...@au...>]

Hi Antonio,

Thank you very much for the quick reply. The symmetry that phonopy finds is indeed the same as in the DFT calculation. If I set FC_SYMMETRY = .TRUE., I see no drift in the force constants after symmetrization. I will proceed with that.

Thank you again,
Sam


On Aug 9, 2023, at 9:24 AM, Antonio Cammarata <cam...@fe...> wrote:

Hello,

the drift may arise when the force constants do not fulfill all the symmetry requirements. Try to set FC_SYMMETRY = .TRUE. when you create the FORCE_CONSTANTS file from the FORCE_SETS. Also, check that the symmetry that phonopy finds for the reference structure is the same that the DFT code finds during the relaxation.

I hope this helps.

Best

Antonio

On 09. 08. 23 14:53, Greene, Samuel wrote:
Hello,

I understand that the “Max drift of force constants” reported by phonopy is an indication of how accurate the underlying DFT calculations are. I’ve been running some calculations in which the magnitude of the max drift seems quite large, at least relative to other examples I’ve seen online. In my calculations, the magnitude is between 0.2 and 1.3. I’ve tried increasing the accuracy of my DFT calculations (including the structure relaxations I perform before running phonopy) by increasing the k-point sampling density and plane wave basis energy cutoff, but this does not seem to help. In some cases, it increases the magnitude of the max drift. Similarly, increasing the supercell size in phonopy helps in some cases but not in others.

Should I be concerned about these large magnitudes of the max drift, and if so, what can I do to try to reduce them?

Thanks in advance for your help,
Sam
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Antonio Cammarata, PhD in Physics
Associate Professor in Applied Physics
Advanced Materials Group
Department of Control Engineering - KN:G-204
Faculty of Electrical Engineering
Czech Technical University in Prague
Karlovo Náměstí, 13
121 35, Prague 2, Czech Republic
Phone: +420 224 35 5711
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ORCID: orcid.org/0000-0002-5691-0682<http://orcid.org/0000-0002-5691-0682>
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