[Pw_forum] relax question

[xu yuehua]

thanks for  all your replies.i shall try it according to your advices.

2007/7/15, Stefano Baroni <baroni at sissa.it>:
>
> Hi All:
>
> a small further comment to explicitly stress a point that has already been
> raised more or less implicitly.
>
> One runs simulations not just for the sake of it, but to calculate
> properties, right? And properties are numbers, right?
>
>
> Well, the right questions to ask at the start of a simulation concern the
> influence of the input parameters on the *specific numbers* one wants to
> calculate (i.e. the *specific property* one wants to study). Of course,
> the energy is a very important property, and it is very correct to ask
> oneself what the influence of the input parameters on the accuracy of the
> calculated energy would be (not to mention its conservation, if one runs a
> MD simulation). It is not the only one, though. If you want to study the
> equilibrium geometry, for instance, it is the accuracy of bond lengths and
> bond angles that would matter, and the softer the system, the more difficult
> it will be to achieve a given target accuracy (this point has been discussed
> in this very thread, I think).
>
>
> All in all, I would recommend that before asking what value a given
> parameter should be given, one should ask oneself "why should I care about
> this parameter?". A proper answer to the latter question will almost
> invariably also answer the first.
>
>
> I understand that the usage of canned packages such as QE or vasp
> encourages a somewhat uncritical approach to the many, many, parameters
> needed to fine tune a well done simulation. This is one more reason, I
> think, to make an effort to ask sometimes general questions before specific
> ones.
>
>
> Cheers - Stefano
>
>
>  On Jul 15, 2007, at 5:29 AM, Xunlei Ding wrote:
>
>  Hi,
>
>
> In the vasp forum, one set the parameter like this:
> EDIFF: 1E-07
> EDIFFG: -0.0005
>
>
> And in the reply it is said, "Anyways, your ediffg is very low. A strict
> force
> criteria is -0.001, and many people would use -0.01 or higher. If you look
> at
> the change in the energy as you go from a force of 0.01 eV/Ang to 0.001eV/Ang,
> it is usually negligible."
>
>
> You can check the difference of results when you set force_conv_thr as
> 1.d-3 and
> 1.d-4. If the difference is negligible, why use too strict threshold?
>
>
> Yours,
> Ding Xunlei
>
>
>
>
> Quoting xu yuehua <njuxuyuehua at gmail.com>:
>
>
>  ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
> [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
>
> Please, if possible, don't  send me MS Word or PowerPoint attachments
> Why? See:  http://www.gnu.org/philosophy/no-word-attachments.html
>
>
>
>
>
>
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>


-- 
Xu Yuehua
physics Department of Nanjing university
China
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