# [Pw_forum] force

Stefano Baroni baroni at sissa.it
Sun Aug 10 11:54:44 CEST 2008

```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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