The calculations of phase transition TERMINATED due to the imaginary frequencies

[Sssss-jg]

Dear all,

I am currently developing my research topic on phase transitions based on the SSCHA 2026 school tutorials (namely Lecture2 and Hands-on-session2). But when I performed the CONFIGS convergence test, the job terminated with the following warning:

RuntimeWarning: invalid value encountered in sqrt a[:] = np.sqrt(1.0 / (2.0 * w))

I suspect that minimizer.run() generated imaginary phonon frequencies, which caused the calculation to fail.

To investigate this, I debugged the script and confirmed that imaginary frequencies were indeed present after minimizer.run() by adding
w_check, _ = minimizer.dyn.DiagonalizeSupercell() if np.any(w_check <= 0): print(f"[Warning] {nconf} configs has negative frequencies: {w_check[w_check<=0]}") # Save the refined auxiliary dynamical matrix refined_prefix = os.path.join(OUTPUT_DIR, f"refined_dyn_{nconf}_") minimizer.dyn.save_qe(refined_prefix)

I tried to explicitly force the use of minimizer.dyn.ForcePositiveDefinite() and minimizer.dyn.Symmetrize() after the minimizer.run() and run the script again, but nothing changed.

Why is this happening? I expected ForcePositiveDefinite() to eliminate the imaginary modes. Is there something I might be missing, or is there a recommended way to resolve this issue? Any suggestions would be greatly appreciated.

I have uploaded the script 6_hessian_converg_nconf.py for your reference.

I look forward to any comments or suggestions. Thank you in advance for your help again.

Jianguo Si
DIMST, CAS

[mesonepigreco]

Hi, it seems very strange.
I suspect the error is not due to imaginary frequencies, in this case it should be written very clearly in the output. The check you are doing on imaginary frequency can just spot a very slightly imaginary part on acoustic modes, that is completely normal and not an issue (even after imposing the ASR). Check the frequencies of the final dynamical matrix inspecting them: where are these imaginary modes? If the code crashes, there should be an error (not just a warning). What is the error? If it crashes without errors, the most likely cause is that you just run out of memory for computing the hessian. Does the terminal says that the process has been killed?