[Pw_forum] stress in CP vs PW, previously no subject

[Konstantin Kudin]

> The numbers with pw.x also change little change. Then I have a
> significant 
> difference in the pressure of 29 kbar with pw vs -100 GPa with CP.
...

>   nfi    ekinc  temph  tempp        etot      enthal       econs     
>  econt    
> vnhh   xnhh0    vnhp   xnhp0
>   220  0.00490    0.0    0.0 -2097.04257 -2097.04257 -2097.04257
> -2097.03768  
> 0.0000  0.0000  0.0000  0.0000
> This was with the default values of ecfixed, etc. The corresponding
> pwscf 
> energy (occupation=fixed) is -2097.047 Ha.

 Well, the biggest problem here is that the CP wavefunction appears to
be unconverged. Have you actually tried to converge it fully? There is 
an option now, electron_dynamics='cg', which should take the guesswork
out of the CP convergence.

 Remember, that stress is the 1st derivative of the energy, and any
noise in the density will be greatly multiplied when taking
derivatives.

 Kostya

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