[Pw_forum] Fw: relax is too slow

[vega]

Dear sir,

Thank you. Now, I'm better understanding on this issue.

vega

=================================================================================
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 8:40 PM
To: PWSCF Forum 
Subject: Re: [Pw_forum] Fw: relax is too slow




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

  =================================================================================
  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
    =================================================================================
    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


  Please, if possible, don't  send me MS Word or PowerPoint attachments
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