[Wien] a question about forces

[Laurence Marks]

What you are finding is "right". When you have small RMTs, here more
the O than anything else, there is a large discontinuity of the
gradients at the RMT (this is implicit in an APW+lo method) unless one
uses very, very large RKmax (e.g. for instance 4.5 here). This changes
exactly how much charge there is within the RMT, and hence the
screening of the potential. When you change the RMTs you do change
this, so the forces and exact interatomic distances will change.

If you re-minimize with your new RMTs (if using PORT, use the
.min_hess from before) you will find that the change in the atomic
positions will in fact be very small. The O-H bond is a strong one, so
a force of 20 mRyd/au will almost certainly only change the bond
distance by 0.01 au or less.

Hence one can only ever compare calculations at the same level of
approximation, where "approximation" is everything from the RMT to the
number of k-points per unit volume.

N.B., I think setrmt tends to make the "H" too large, and personally I
would tend to make it smaller (e.g. 0.5) and make the O larger. You
should also check that the RMT for the B is large enough as you can
lose core electrons if you are not careful. The core states for B are
wider than those for O as Z is smaller.

On Fri, Jan 13, 2012 at 9:05 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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-- 
Professor Laurence Marks
Department of Materials Science and Engineering
Northwestern University
www.numis.northwestern.edu 1-847-491-3996
"Research is to see what everybody else has seen, and to think what
nobody else has thought"
Albert Szent-Gyorgi