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