[Pw_forum] Fw: relax is too slow

[Stefano Baroni]

On Sep 13, 2008, at 2:20 PM, vega wrote:

> Dear sir,
>
> Thank you for your responding.
>
> Could you tell me what is the most important thing to judge whether  
> the accuracy of a numerical calculation is enough.
> The energy? force? atomic coordinates? or any other things?

It depends on what you are interested in, of course. What you are  
interested in (geometry, energy, other properties ...) is what you  
should look at ... for instance, it is known that obtaining well  
converged values for the dielectric constant may require k-point  
meshes way finer than required for a plain energy calculation. Here as  
well as in many other circumstances, the correct answer is not one  
that the beginner may like to hear: "it depends" ...

life is beatiful and hard. if it were not as hard, it would not be as  
beautiful

cheers - SB

>
> vega
>
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> ======================================================================
> Vega Lew (weijia liu)
> PH.D Candidate in Chemical Engineering
> State Key Laboratory of Materials-oriented Chemical Engineering
> College of Chemistry and Chemical Engineering
> Nanjing University of Technology, 210009, Nanjing, Jiangsu, China
>
> From: Stefano Baroni
> Sent: Saturday, September 13, 2008 6:04 PM
> To: PWSCF Forum
> Subject: Re: [Pw_forum] Fw: relax is too slow
>
> the same way as you would choose any other parameter controlling the  
> accuracy of a numerical calculation. try a value; try a few others  
> corresponding to increasing accuracy, and stop when you see that the  
> increase in the accuracy is not worth the extra workload. never  
> forget to use common sense (whenever it is available).
>
> SB
>
> On Sep 12, 2008, at 11:36 AM, vega wrote:
>
>> Dear sir
>>> vega wrote:
>>>
>>>> As far as I tested, the energy would be within 1% for large  
>>>> supercells,
>>>> but the coordinates of several atoms may differ 200%.
>>>
>>> doesn't make sense
>>
>> could you tell me more about how to choose the k point mesh?
>>
>> vega
>> = 
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>> = 
>> =====================================================================
>> Vega Lew (weijia liu)
>> PH.D Candidate in Chemical Engineering
>> State Key Laboratory of Materials-oriented Chemical Engineering
>> College of Chemistry and Chemical Engineering
>> Nanjing University of Technology, 210009, Nanjing, Jiangsu, China
>>
>> --------------------------------------------------
>> From: "Paolo Giannozzi" <giannozz at democritos.it>
>> Sent: Friday, September 12, 2008 5:27 PM
>> To: "PWSCF Forum" <pw_forum at pwscf.org>
>> Subject: Re: [Pw_forum] Fw: relax is too slow
>>
>>
>>> -- 
>>> Paolo Giannozzi, Democritos and University of Udine, Italy
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>
> ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
> Trieste
> http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
> stefanobaroni (skype)
>
> La morale est une logique de l'action comme la logique est une  
> morale de la pensée - Jean Piaget
>
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