[Wien] difference of min- energy point and zero -force point

[Stefaan Cottenier]

>   3)I use RK=7  and mesh 18X18X18  for bulk and 10X10X1 for   
> asupercell  with 38 atoms and a surface cell(2X2) .I study about   
> work function and I use from this parameters because :a)give me good  
>  value  for  workfunction  and   b)for  avoid of  computational cost  
>  of large parameters.

Don't take this personally, but these are very bad and even dangerous  
arguments. As computational physicists, we MUST NOT take the  
parameters that give us good agreement with experiment. We must take  
the parameters that give us numerically accurate results within the  
framework we work in (e.g. DFT-LDA). If we don't do that, we're just  
doing meaningless fitting, not physics. If it turns out you find the  
computational cost is too much for the quantity you want to study,  
then so be it. Put the problem aside and do something else instead of  
doing something meaningless. In your case, this means you must verify  
whether going to a larger RKmax or fine k-mesh does not change you  
work function to an amount that would be large enough to alter your  
physical conclusions. As long as you are not sure about this, there is  
no point in proceeding with the rest of your work.

There is a good reason why I spent some time writing this down: a bad  
approach to DFT studies affects the entire community. Imagine you come  
to some conclusion based on an unjustified small basis set. Perhaps  
you'll even make a prediction based on your results. Then an  
experimentalist tries to verify your prediction, and find it to be  
wrong. Obviously, the conclusion of his or her paper will be: DFT  
gives bad predictions. If this happens too often, then DFT gets an  
unjustified bad image, and few people will believe DFT predictions in  
the future, even if they are made with good basis sets. Our entire  
community has a responsibility in keeping up the credibility of DFT:  
there are enough places where it doesn't agree with experiment, we  
should not create additional ones by being too tolerant on numerical  
issues as basis set and k-mesh.

I hope you seriously think about this before continuing!

Now back to your problem:

As you apparently did not test the accuracy of your forces with  
respect to k-mesh and basis set, it might be that you are working with  
too much numerical noise on the forces, leading mini to an incorrect  
result. So test the accuracy of your forces.

This is an interesting issue:

> with attention to this  point  that min-energy is obtained  in  
> smaller time by plot total  energy  versus  distance of atom from  
> surafce    with respect to use  of mini..

As the force is the gradient of the total energy surface, this  
statement cannot be true. Even if you don't use mini and displace the  
atoms yourself, it is worthwhile to calculate the force in the last  
iteration and plot not :ENE but the force as a function of  
displacement. It will show you much better in which direction and by  
which amount you have to displace, requiring less steps to find the  
equilibrium than if you use only the energy information. As a check,  
of course, you can look at the energies afterwards. If your numerical  
precision was OK, the zero of the force and the energy minimum must  
agree.

Stefaan


Quoting naser zare <naser_zd2000 at yahoo.com>:

> Dear Stefaan,
>   Thank you very much for your reply.
>   I should mention that:
>   1)the discrepancy in position is 0.122A(1.544A for  min-energy and  
>  1.666 A for zero-force)
>   2)convergence criteria for energy is  0.0001ry and for charge is   
> 0.002 and when I use from
>   mini for force is 4 mry/au
>   3)I use RK=7  and mesh 18X18X18  for bulk and 10X10X1 for   
> asupercell  with 38 atoms and a surface cell(2X2) .I study about   
> work function and I use from this parameters because :a)give me good  
>  value  for  workfunction  and   b)for  avoid of  computational cost  
>  of large parameters.
>   4)when I used mini , I  move only one atom (  like when I obtain   
> min of energy ) and all atoms of slab are fixed.
>
>   I have a question,can I  fell sure that point obtained from plot   
> of total energy  versus distance  of atom from surafce  is true   
> point and my problem is calculation of force?with attention to this   
> point  that min-energy is obtained  in smaller time by plot total   
> energy  versus  distance of atom from surafce    with respect to use  
>  of mini..
>
>     thank you  very much for your attention.
>  Best Regards.
>  Nasser Zare Dehnavi
>
>
> Stefaan Cottenier <Stefaan.Cottenier at fys.kuleuven.be> wrote:
>
> You need to provide more information, for instance:
>
> * How big is the discrepancy in position?
> * How well are your calculations converged and what are your
> convergence criteria?
> * Did you verify your basis set and k-mesh are well enough to provide
> good forces?
> * When you used mini, did you move only one atom, or all of them?
> *...
>
> Stefaan
>
>
>
> Quoting naser zare :
>
>> Hello
>> Dear all:
>> I use from Wien2k 6.4. I want to obtain position of adsorption of
>> atom on a surface.I do it in two way :1)I move atom towards
>> surface step by step and I plot total energy
>> versus distance from surface and I obtain point of min energy and
>> 2)I relax position of atom by MINI, but these two point are not
>> the same.on the other hand ,point that obtained by Mini has not
>> lowest energy. I want to know reason of this problem .thank you
>> very much for your attention.
>> Best Regards.
>> Nasser Zare Dehnavi
>>
>>
>>
>>
>> ---------------------------------
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>
>
> --
> Stefaan Cottenier
> Instituut voor Kern- en Stralingsfysica
> K.U.Leuven
> Celestijnenlaan 200 D
> B-3001 Leuven (Belgium)
>
> tel: + 32 16 32 71 45
> fax: + 32 16 32 79 85
> e-mail: stefaan.cottenier at fys.kuleuven.be
>
>
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-- 
Stefaan Cottenier
Instituut voor Kern- en Stralingsfysica
K.U.Leuven
Celestijnenlaan 200 D
B-3001 Leuven (Belgium)

tel: + 32 16 32 71 45
fax: + 32 16 32 79 85
e-mail: stefaan.cottenier at fys.kuleuven.be


Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm