[Pw_forum] force

[Stefano Baroni]

Dear Ali,

first of all, let me rejoice for receiving scientific correspondence  
from such a troubled part of the world, and let me express the wish  
that having the state of mind to ask serious scientific questions is a  
promise of peace.

Coming to your question. The correct answer is a very frequent one in  
science: "it depends". Let me explain what I mean by this. I suppose  
you want to know the force threshold below which an energy  
minimization should be stopped. Let us ask ourself, first, what enegry  
minimizations are meant for. If they are meant to find equilibrium  
geometries, then the first thing you should ask is to which precision  
you want to know the interatomic distances. This only you can say (it  
depends on the accuracy the data are known experimentally, or the  
further use you may want to make of these calculated data). Then, you  
have to ask what maximum error in the force is compatible with the  
error that you judge tolerable in the distances. The answer depends on  
the stiffness of the material (or, technically, on the magnitude of  
the interatomic force constants), that you have to know (explicitly or  
implicitly), at least as an order of magnitude. If you do not have the  
faintest idea of how stiff the material is, the rule of thumb is: form  
an idea by yourself, and do not rely on "internet rumors". An  
inexpensive way of forming your own ideas is to do some computer  
experiment. Do a calculation with some force threshold. Repeat the  
same calculation with a threshold ten times as small. How much do the  
atomic positions change? Repeat the same game for different thresholds  
and for different, but related, systems. You will soon see what is  
best in your specific case. Little by little, you will develop your  
own rules of thumb. If they are good, they will not be general. You  
will see that the same threshold would apply for the majority of  
cases, but you will also discover exceptions. Many people here would  
suggest that a threshold in between 0.1 and 1 mRy/au is adequate in  
many circumstances, but this suggestion will never surrogate your own  
experience.

Hope this helps a little bit.
Salam aleykum - Stefano Baroni


On Aug 9, 2008, at 9:31 PM, ali jalaukhan wrote:

> Dear All
> I want to know about the suitable value for force-convergence  
> threshold .Is .002 0r 0.003(Ryd/a.u) appropriate or not ?
> thanks
> Ali jalaukhan
> karbala university,Iraq
>
>
>
>
>
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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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