[Pw_forum] vc-rx

Gabriele Sclauzero sclauzer at sissa.it
Wed Apr 15 13:31:31 CEST 2009

Mehrnaz Anvari wrote:
> Hell
> I read your answers, but I think (it maybe wrong)  after density 
> calculating  in each step of vc-rx calculations, the program should use 
> it in the next step. 

I don't know much about vc-rx calculation, but I can tell you for sure that in the relax 
calculations it does. The self-consistent charge density at the previous relaxation 
step(s) is(are) saved and used as initial guess for the next step.

> so because of trivial changes in cell parameters 


> the number of iterations should decrease in each step, shouldn't?

In principle it should, but if the atomic configuration changes a lot from a step to the 
other (as it can happen at the beginning of the relaxation) this cannot be guaranteed.

>   I have another question about introducing one layer to code. How could 
> I select c parameter along z direction ,I mean how large it should be? 

In principle with the option cell_dofree you could chose which of the lattice parameters 
you want to relax and which to keep fixed at the value specified on input. From previous 
posts I suspect this option is not always working as expected. You can try it and report 
problems, and possibly try/help to fix them if you are really interested in using it.

> Could I say if stress along z direction become zero after 2-3 steps  in  
> vc-rx  my selection is correct?

At the end of a variable cell calculation all stress components should be below the 
convergence threshold.
If you want a lattice parameter to be fixed at a value other than the equilibrium one, 
either you try cell_dofree or implement constraints on cell parameters...
Maybe I don't understand exactly what is your aim.


> Best Regards
> M.Anvari
> Iran University Of Science & Technology
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