[Pw_forum] A question about Total force

[Guido Fratesi]

Dear Payam,

what you report is consistent with a too high threshold for the 
self-consistency loop (ie forces are not computed accurately enough): 
try reducing conv_thr (say to 1d-8) or use upscale.

Also have a look to the total energy: is that decreasing, while forces 
are fluctuating? Then, the system is still relaxing towards its minimum 
even though forces are small.

HTH,
Guido

PS
Notice that it occurs sometimes that the system relaxes in one 
direction, reducing the force quite rapidly. Then the algorithm finds 
new possibilities to further reduce the total energy, yet the forces are 
small. An example of that might be when you put a strongly deformed 
molecule close to a surface, above an adsorption site which is not the 
optimal one. First, the molecule relaxes more or less where you placed 
it, then it finds its way towards another adsorption site (if symmetry 
allows it to do so).


Il 03/15/2012 04:25 PM, Payam Norouzzadeh ha scritto:
> Dear QE users
>
> I usually look at total force to check if my structure is being relaxed
> or not.Most of times it reduces to a certain level (~0.003) continuously
> and
> then starts to fluctuate. I mean it reverses the trend of reducing total
> force.Simply total force begins to grow and sometimes never gets smaller.
> The question is that how can I push QE to reduce total force
> continuously when I am trying to relax a structure?Why total force
> fluctuates?
>
> Best regards,Payam Norouzzadeh
>
>
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-- 
Guido Fratesi

Dipartimento di Scienza dei Materiali
Universita` degli Studi di Milano-Bicocca
via Cozzi 53, 20125 Milano, Italy