[Pw_forum] The recalculated pressure is beyond the range specified by press and press_conv_thr

[stefano de gironcoli]

the last two pressure values printed
total   stress  (Ry/bohr**3)                   (kbar)     P= 0.08
total   stress  (Ry/bohr**3)                   (kbar)     P= 3.17
are the pressure as computed by the code at the end of the vc-relaxation 
with the cutoff sphere deformed according to the cell evolution and the 
pressure computed for the SAME ionic configuration but with the cutoff 
sphere reset to the proper shape.
If the cutoff is high enough (or if the deformation of the cell) is 
small enough these two values should be close.
Increasing the cutoff should make things better.
Alternatively (especially if the cell contracts wrt the initial value) 
one can experiment with the ecfixed,qcutz,q2sigma variables.

stefano

On 07/06/2017 16:45, Tsung-Lung Li wrote:
> Dear QE Friends:
>
> Below is an excerpt of my recent PW simulation,
>
> ----------------------------------------
> &control
>     :
>     :
>     calculation = 'vc-relax'
>     restart_mode = 'from_scratch'
>     etot_conv_thr = 1.0D-4
>     forc_conv_thr = 1.0D-3
> /
> &ions
>     ion_dynamics = 'bfgs'
> /
> &cell
>     press = 0.001D0  ! kbar
>     press_conv_thr = 0.2D0  ! kbar
> /
> ----------------------------------------
>
> The simulation converges.
>
> A final scf calculation at the relaxed structure.
> The G-vectors are recalculated for the final unit cell.
>
> The last few lines containing "P=" are
>
> grep "P=" t.log
>
> total   stress  (Ry/bohr**3)                   (kbar)     P= 0.13
> total   stress  (Ry/bohr**3)                   (kbar)     P= 0.19
> total   stress  (Ry/bohr**3)                   (kbar)     P= 0.08
> total   stress  (Ry/bohr**3)                   (kbar)     P= 3.17
>
> The recalculation suggests a final pressure of 3.17 kbar, which is
> BEYOND the specified range of
> [0.001D0-0.2D0, 0.001D0+0.2D0] = [-0.19900, 0.20100].
>
> Does this mean that the simulation does NOT converge to condition specified
> in &cell section?  Or, do I make any mistake?
>
> Thank you in advance.
>
>
> Best Regards,
> Tsung-Lung Li
>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum