Hello dear researchers
I am beginner in Abinit software, I made a simulation of piezoelectric properties and I got its two errors. Please if you have any idea how I can solve them please.
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ddb_piezo : WARNING -
Acoustic sum rule violation met : the eigenvalues of accoustic mode
are too large at Gamma point
Increase cutoff energy or k-points sampling.
The three eigenvalues are: 2.589265E-05 2.380597E-05 2.589265E-05
ddb_piezo : WARNING -
Unstable eigenvalue detected in force constant matrix at Gamma point
The system under calculation is physically unstable.
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This looks like you haven't done a good geometry optimisation for your structure yet. You need to make sure the forces and stresses are "zero" to a good tolerance.
Phonon calculation also need a good cut-off energy, density grid and k-point sampling, but the phonon calculations are time-consuming so doing the convergence the obvious way is computationally expensive. A good proxy property is the stress on the simulated cell, so I recommend that you do the convergence tests for single energy calculation of a fixed cell, and use the cell stress convergence to determine the cut-off energy etc.
Hope that helps,
Phil Hasnip
(CASTEP Developer)
Thank you sir Philip James Hasnip for your answer.
What I understood is that I have to do the convergence test before doing the optimization of the structure (to determine the cut off) and then I do the optimization (I have to choose a tolerance until I am sure that the forces and stresses are gone =0)?
Hamza Elaamri yes, that's right. You determine a good cut-off energy, k-point sampling and density grid first, and then do a full variable-cell geometry optimisation.
Only once you have a good, well-converged geometry can you do phonon calculations reliably, without seeing "negative" (imaginary) modes.
Philip James Hasnip Thank you sir.
I know how to determine a good cut off and k-points, but for density grid how can I determine it???
Thank in advance
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