[QE-users] vc-md simulation: Pressure stays below target

[Kazume NISHIDATE]

Dear Seitaro Yazawa,

> Even though I’ve set the target pressure to 0 GPa for 2000 steps, the time-averaged pressure over 
> several thousand steps stays about 0.5 GPa lower than expected. I also checked using a moving  

In general, the fluctuation of the pressure, say, plus or minus 0.5 GPa, is normal in finite temperature molecular dynamics simulations as the ‘virial’ is used to derive the macroscopic pressure from the atomistic forces. Imagine that the 80 atoms moving correspond to the temperature of 300 K, and recording their movement every femtosecond. The fluctuations of microscopic forces directly affect the calculated instantaneous ‘virial’ pressure.

> So far, I’ve tried several approaches to reduce Pulay stress and improve pressure convergence:
> Increased the plane-wave cutoff energy to 80 Ry

Instead, decrease the setting temperature to 50 K and calculate the system for an extended period, such as 100000 steps, to see if it achieves the desired pressure.

Sincerely yours,


西館数芽
Kazume NISHIDATE Ph.D

Department of Systems Innovation Engineering,
Graduate School of Science and Engineering, Iwate University
4-3-5 Ueda, Morioka, Iwate, 020-8551 JAPAN
Phone:+81-19-621-6391
kazume.nishidate at gmail.com, nisidate at iwate-u.ac.jp
https://sites.google.com/site/nisidatelab/