[QE-users] CPMD Stability Analysis at Different Temperatures for Li-Vacancy System

[Sharma Rahulkumar Rajkumar]

"Dear users, I would be grateful if you could kindly guide me on the
following query."

I am performing CPMD simulations of a solid electrolyte containing a Li
vacancy in the NVT ensemble using cp.x(Quantum ESPRESSO). I run MD at
various temperatures (500, 600, 800, 900, and 1100 K) using the Nose–Hoover
thermostat and fnosep to control temperature fluctuations.

At higher temperatures (e.g., 900 K), I observe a drift in key parameters
such as electron fictitious kinetic energy (ekinc), total energy (etot),
conserved energy (econs), and thermostat energy (econt). Temperature
control is also less stable at high temperatures (which I suspect may be
improved by increasing fnosep).

I currently use emass = 200, dt = 1, and fnosep = 35.

For reference, I have attached plots showing the trends of key simulation
parameters at 300 K and 900 K for comparison.

My queries are:

   1.

   For a system with a Li vacancy, what are the key criteria to monitor for
   a physically valid CPMD run? How should ekinc, econs, and other
   parameters behave over time? In the paper (Fig 3), DOI:
   https://doi.org/10.1103/PhysRevA.44.6334, the authors also observed an
   increase in ekinc for defect systems.
   2.

   Should the same set of tuned parameters (emass, dt, fnosep) be used
   across both low temperatures (e.g., 500 K) and high temperatures (e.g.,
   900–1100 K), or should they be adjusted depending on the temperature?

Thank you for your guidance.
Could you advise how to adjust these parameters to ensure better energy
conservation and simulation stability at higher temperatures ?
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