[Pw_forum] accuracy of stress determination of vc-relax

[Paolo Giannozzi]

Follow variable "press_conv_thr". It is copied to "epsp"
which is then used in PW/src/move_ions.f90 and passed
to subroutine "bfgs", in Modules/bfgs_module.f90, as
"cell_thr". There you find the way the convergence is 
checked. Please report if something is not consistent
with whatthe documentation says

Paolo

On Thu, 2014-12-11 at 08:52 +0200, Uri Argaman wrote:
> Dear QE users and developers
> 
> I do vc-relax with BFGS algorithm as follows:
> 
> &CONTROL
>   calculation = "vc-relax",
>   pseudo_dir  = '/home/uriargaman/espresso-5.0.3/pseudo/',
>   outdir      = '/home/uriargaman/tmp/tmp28898/',
>   tstress=.true.
>   verbosity='high',
>  /
> &system
>     ibrav=  4, celldm(1) =5.548454954360, celldm(3)
> =1.581979631777413 , nat= 2, ntyp= 1,
>     ecutwfc =80, ecutrho=1200, 
>     occupations='smearing', smearing="methfessel-paxton",
> degauss=0.01,
>  /
> &ELECTRONS
>   conv_thr    = 1.D-9,
>   electron_maxstep=1000
>   mixing_beta = 0.7D0,
> /
> &IONS
>    ion_dynamics="bfgs",
> /
> &cell
>   cell_dynamics="bfgs",
>   press=0.6,
>   press_conv_thr=0.01,
> /
> ATOMIC_SPECIES
>   Ti   47.9  Ti.pbe-sp-van_ak.UPF
> ATOMIC_POSITIONS (crystal)
>    Ti      0.666666666    0.333333333    0.250000000    0  0  0
>    Ti      0.333333333    0.666666666    0.750000000
> K_POINTS {automatic}
>  24 24 24 0 0 0
> 
> 
> In the last iteration (after begin final coordinates) I got stress:
>           total   stress  (Ry/bohr**3)                   (kbar)     P=
> 0.71
>    0.00000485  -0.00000000  -0.00000000          0.71     -0.00
> -0.00
>   -0.00000000   0.00000485   0.00000000         -0.00      0.71
> 0.00
>   -0.00000000   0.00000000   0.00000481         -0.00      0.00
> 0.71
> 
> and the stress in the iteration before (last iteration before begin
> final coordinates):
>   total   stress  (Ry/bohr**3)                   (kbar)     P=    0.73
>    0.00000499  -0.00000000   0.00000000          0.73     -0.00
> 0.00
>   -0.00000000   0.00000499   0.00000000         -0.00      0.73
> 0.00
>    0.00000000   0.00000000   0.00000495          0.00      0.00
> 0.73
> 
> I did'nt reach the pressure threshold although I have very good
> convergence with respect to all numerical parameters. The threshold is
> achieved only for hydrostatic condition but not for the value of the
> stress (P=0.73 kbar instead of 0.6 kbar). Is the parameter
> "press_conv_thr" is just for the hydrostatic condition and not for the
> absolute value of the stress? How can I get more accurate stress?
> 
> Thank you very much
> 
> Uri Argaman
> 
> Ben-Gurion University
> 
> Israel
> 
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-- 
 Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
 Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
 Phone +39-0432-558216, fax +39-0432-558222