[Pw_forum] vc-relax

[David Farrell]

I have run into convergence issues (fluctuations about a minimum) with  
larger cells and too tight of a force and energy tolerance (yours are  
quite tight).

You may want to back off the tolerances a bit, then restart it with  
tighter tolerances, incrementing until you get the desired accuracy.   
That or only set 1 of the tolerances tight. Since both criteria need  
to be met, you may be over-constraining the system and so you get  
fluctuations. One approach may be to back off the energy tolerance, so  
that the forces are tightly converged (which often occurs after the  
energy has converged well)

Or I could be completely wrong.

Dave

On Jan 24, 2009, at 4:48 AM, hania djani-ait aissa wrote:

> Dear all,
> i am doing calculation on orthorombic system containing 36 atoms  
> (Bi2WO6). My computer is a workstation HP xw6400 quad-core Xeon,  
> 2GB. Vc-relax on this system spent more than 01 mounth and still not  
> finish. the energy is converging but  forces fluctuate. is there  
> something wrong in my input in terms of umass or dt? or have i to  
> capitulate to this very slow convergence?
> here is the input file. Any suggestions will welcome.
> &control
>     calculation  = 'vc-relax'
>     restart_mode = 'from_scratch'
>     pseudo_dir   = '/root/pwscf/pseudo/'
>     outdir       = '/root/tmp/'
>     tstress      = .TRUE.
>     tprnfor      = .TRUE.
>     nstep        = 55
>     etot_conv_thr= 1e-5,
>     forc_conv_thr= 1e-4,
>     dt           = 30,
>  /
>  &system
>     ibrav=8
>     celldm(1)=10.37,
>     celldm(2)=0.996,
>     celldm(3)=3.00,
>     nat=36
>     ntyp=3
>     ecutwfc=35.0
>     ecutrho=200.0,
>
>  /
>  &electrons
>     conv_thr = 1e-8,
>     mixing_beta=0.7,
>  /
>  &ions
>  /
> &cell
>     cell_dynamics = 'damp-w',
>     press         = 0.00,
>     wmass         = 0.004,
> /
> ATOMIC_SPECIES
>
>   W     183.84    W.pz-bhs.UPF
>   Bi    208.98    083-Bi-ca-d-vgrp_ji.uspp.UPF
>   O     15.9994   O.pz-van_ak.UPF
>
> ATOMIC_POSITIONS alat
> W        0.000000000   0.005016973   0.747761262  0 1 1
> W        0.500000000   0.990983027   2.252238738  0 1 1
> W        0.500000000   0.492983027   0.747761262  0 1 1
> W        0.000000000   0.503016973   2.252238738  0 1 1
> Bi       0.964931181   0.523336245   1.268793038
> Bi       0.464931181   0.472663755   1.731206962
> Bi       0.464931181   0.970663755   1.268793038
> Bi       0.964931181   0.025336245   1.731206962
> Bi       0.981726782   0.473760364   0.230569958
> Bi       0.481726782   0.522239636   2.769430042
> Bi       0.481726782   0.024239636   0.230569958
> Bi       0.981726782   0.971760364   2.769430042
> O        0.100459245   0.081203101   0.435231801
> O        0.600459245   0.914796899   2.564768199
> O        0.600459245   0.416796899   0.435231801
> O        0.100459245   0.579203101   2.564768199
> O        0.264260826   0.262143525   3.000064967
> O        0.764260826   0.733856475  -0.000064967
> O        0.764260826   0.235856475   3.000064967
> O        0.264260826   0.760143525  -0.000064967
> O        0.247153777   0.235070329   1.501190932
> O        0.747153777   0.760929671   1.498809068
> O        0.747153777   0.262929671   1.501190932
> O        0.247153777   0.733070329   1.498809068
> O        0.283152650   0.701520345   0.680189839
> O        0.783152650   0.294479655   2.319810161
> O        0.783152650   0.792479655   0.680189839
> O        0.283152650   0.203520345   2.319810161
> O        0.335776234   0.239505837   0.816308565
> O        0.835776234   0.756494163   2.183691435
> O        0.835776234   0.258494163   0.816308565
> O        0.335776234   0.737505837   2.183691435
> O        0.586065178   0.587540848   1.058853940
> O        0.086065178   0.408459152   1.941146060
> O        0.086065178   0.906459152   1.058853940
> O        0.586065178   0.089540848   1.941146060
>
>
>  K_POINTS {automatic}
>   5 5 5 1 1 1
>
> thanks a lot
>
> Hania Djani-ait aissa
> PHD student
> Algeria
>
>
>
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David E. Farrell
Post-Doctoral Fellow
Department of Materials Science and Engineering
Northwestern University
email: d-farrell2 at northwestern.edu

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