[Pw_forum] big atomic forces

[Stefano Baroni]

Others may have better answers than me, but ...

1) I do not know how forces are calculated with tetrahedra, but I  
think fear that the total energy calculated with tetrahedra is not  
variations, so that Helmann Feynman forces may have some problems ...

2) Not that a force of 1d-3 is that large after all ...

As said, other answers may be more illuminating than mine.

Stefano

On Jun 4, 2007, at 7:24 PM, Jess Kondor wrote:

>
> Hello, everybody,
>
>  I have a question. I relaxed a surface using 8x8x1 k-point mesh  
> and 'm-v' smearing method with degauss=0.01 with criteria for  
> forces 4.d-4 . Then I performed 'scf' calculation with increased  
> number of k-points (12x12x1) and turn on an option 'tprnfor  
> = .true.' in order to calculate forces. Forces are OK (all lower  
> than 4.d-4). But when I perform 'scf' with occupation method  
> 'tetrahedra'  (other parameters are the same), forces on atoms are  
> much higher (some of them are ~2.0d-3). Does it mean that geometry  
> relaxation is not been completed?
>
> cheers,
>    Jess
>

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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