[Pw_forum] A QUESTION about the precision of relaxation

[Stefano Baroni]

Dear Yuehua:

On Nov 12, 2007, at 4:00 AM, xu yuehua wrote:

> hi everyone ,
> i am now do a relax job.my uint cell is a supercell . and i only  
> want to relax a direction of lattice ,for example c .
> so i tried to vary the c .and compute. see the stress.but i do not  
> know how to adjust the c on the base of the result like:
>  entering subroutine stress ...
>
>           total   stress  (Ry/bohr**3)                   (kbar)      
> P=   -1.06
>   -0.00000893  -0.00000001  -0.00000001         -1.31       
> 0.00      0.00
>   -0.00000001  -0.00000892   0.00000000          0.00     -  
> 1.31      0.00
>   -0.00000001   0.00000000  -0.00000382          0.00      0.00      
> -0.56
>
>
> (10there is "-0.56" and the "-" stands for .i should  the reduce c?

a positive pressure (or a positive component of the stress, for that  
matter) will make the material expand, if unconstrained.
>  (2)0.56 is a large stres?
>

the answer is "NO", but this is an answer you could have answered  
yourself. assume you know the elastic constants of your material (if  
you don't, just take the experimental values, or any reasonable  
guess). how much would c vary given the known values of the elastic  
constants and of the zz componenty of the stress?  unless the  
material is exceedingly soft, you will find that c will vary very  
little. you can practice this yourself. just let c vary by, say, 5%,  
and see by how much would \sigma_{zz} vary ...

> (3) is there any other way to determine the suitable c?
>
calculate and tabulate E vs. c and find the minimum. beware, though,  
that the curve that you will obtain should be rather rugged. this  
issue has been discussed several times in several places, including  
this forum, so I will stop here ...
> thank you very much if you could give a advice .
>

you are most welcome
Stefano Baroni

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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