[Pw_forum] Relaxation of low symmetry lattices

[Stefano Baroni]

On Oct 9, 2009, at 9:46 AM, Lorenzo Paulatto wrote:

>
> On Fri, October 9, 2009 09:26, Manoj Srivastava wrote:
>> which is one order of magnitude smaller than before :) but the KE  
>> cutoff
>> is ridiculously high! I also increase k points with keeping KE  
>> cutoff 50
>> Ry, and that gave me -
>
> Your initial choice of k-point grid is not very wise: as you cell is  
> much
> longer in the x and y direction than in the z direction you will  
> need more
> points in z than in x and y. I would say about 2 - 3 times as many.  
> e.g.
> you could choose:
> K_POINTS automatic
>   6 6 14  0 0 0
>
>> Gabriele: Would you mind to tell me how can i get rid of FFT
>> incompatibility issue?
>
> I just told you, in my previous email. Yet I don't think it's a good  
> idea,
> and the forces won't be zero anyway. Your lattice have lower  
> symmetry than
> the FCC lattice (it is actually a supercell), hence you can only  
> have zero
> forces up to the numerical accuracy.


all in all, "zero" in the physical sciences means essentially two  
conceptually different things

1) a quantity is zero because a physical law forbids it to be  
different from zero. typically, "physical" laws of this kind are not  
quite "physical", but rather "geometrical" (i.e. dictated by symmetry,  
selection rules, and the like).

2) a quantity is _much smaller_ (to within numerical/experimental)  
accuracy than detectable by any physical means.

Forces in an FCC lattice where cubic symmetry is explicitly imposed  
are zero because of 1. If cubic symmetry is not explicitly imposed,  
they may "vanish" (if ever) because of 2. Would you now please tell us  
for which purpose the forces that you calculate (and that do not  
vanish for reasons that you understand now) are too large?

SB

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) /  
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