[Wien] a question about forces

[Laurence Marks]

N.B., a good way to look at this is probably to generate
charge-difference maps. I am almost certain that you will see the
effect clearly. Atoms with small RMTs are tricky in Wien2k.

On Fri, Jan 13, 2012 at 9:33 AM, Laurence Marks
<L-marks at northwestern.edu> wrote:
> 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



-- 
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