[Pw_forum] A QUESTION about the precision of relaxation

[xu yuehua]

Dear Baroni
thanks for your advice .i know how to deal with it.
bests



2007/11/12, Stefano Baroni <baroni at sissa.it>:
>
> 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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>
>
>
>
>
>
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-- 
Xu Yuehua
physics Department of Nanjing university
China
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