[Wien] a question about forces

[Zhou Bing]

Hi David, thank you for your attention and consideration. 
Yes, in both sets of calculations total forces (FOR switch, -fc) were considered.
Any suggestion and comments are highly appreciated!
Have a good day!
Zhou



> 
> Zhou, in running the second set of calculations did you include force convergence (i.e., run_lapw -fc 0.1) in your job script? If not the forces given in scf will be only partial forces (without a valence correction) and would not be expected to correspond to the small forces in your optimized structure. - David Parker
> 
> 
> On 1/13/12 10:05 AM, "Zhou Bing" <umbingz at isl.ac.cn> wrote:
> 
> 
> 
> I am puzzled by a question about forces:
> 
> My case is a mineral (ulexite) with 2 Ca atoms, 2 Na atoms, 10 B atoms,
> 34 O atoms and 32 H-atoms in the unit cell. I have relaxed the atomic
> positions using the following RMT-values: Ca=2.27, Na=2.21, B=1.27,
> O=0.9, H=0.48. The matrix size was 11400 (RKM=2.50), with 3 k-points in
> the IBZ and GMAX=20. All forces ended up below 1 mRy/au.
> 
> After I noticed that these RMT-values were different from the ones
> recommended by setrmt_lapw, I choose the values suggested by the latter:
> Ca=2.15, Na=2.09, B=1.22, O=1.18, H=0.57. Without altering the positions
> and using the same settings as before (including RKM=2.50, now resulting
> in a matrix size of almost 7000 only), the forces became suddenly much
> higher, with values of sometimes more than 20 mRy/au.
> 
> In increased RKM to 3.00 (leading to the same matrix size of 11700 as
> before) and increased the number of k-points to 15: this did not alter
> these high forces in a significant way.
> 
> I need the optimized internal positions, and it is worrying that a small
> (?) change of RMT but an equivalent accuracy leads to forces that are so
> much different. Which of both sets of values can I trust, and why? Is
> there any other parameter that I failed to consider?
> 
> The atoms with these high forces are O and H only (although there are
> changes in the forces on Ca, Na and B as well, but one order of
> magnitude less). Not all of the O atoms, however: twelve of them show a
> significantly smaller change of force than the others.
> 
> Both sets of calculations were cross-checked on several computer
> systems, to exclude any compilation-related problems.
> 
> Does anybody have a clue?
> 
> 
> 
> 
> 
> 
> 
> 
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