[Wien] accuracy test for RKMAX

[santhy kathirvelu]

Dear Prof.Blaha and Stefaan, 
  Thanks a lot.  Your replies are very helpful.  I applied those in my calculations.  Below this I will give the result, just tell that I am in write track.  
   
  By Prof.Blaha’s advice, I added few more negative volumes and then I plotted the results, it gave nice parabola and shows that at -6 only gives the min. energy.  
   
  Then I checked the accuracy test to k-point, I repeated the calculation with 1000, 2000 and 3000 for k point.   I  listed result below,
  k-point             RKM               Enegry                          Volume
  1000                6.93                 -1707.621751              113.36162
  2000                6.99                 -1707.621552              ,,
  3000                6.61                 -1707.621562              ,,
   
  Therefore, for Ti-bcc system with RMT=2.5, RKMAX=7,  RKM=6.93,  GMAX=12 and k-point =1000 gives the optimized structure with volume V=113.36161 (a = 3.226757A) and  E=-1707.621751Ry.  Is it correct?
   
   
  In the above calculation, I used RMT of Ti is 2.5(default).  From the mailing list (site: http://zeus.theochem.tuwien.ac.at/pipermail/wien/2004-April/002424.html)
  I changed the RMT of Ti from 2.5 to 2.69 (because nn-distance = 5.39080).
  I used remaining parameters as same as the above calculation (RKMAX=7, GMAX=12 and k-point=1000).  Using this data, I am able to run SCF, but not volume optimization with negative volumes because RMT is close to nn-distance (positive volumes give higher energy and it is not possible to get a parabola here).  This calculation is almost look like single point energy calculation (i.e. it gives the energy of the given structure) and there is not much energy difference from the above calculation.  I listed the result below.  
   
  RMT=2.69, RKMAX=7, RKM=6.89,  GMAX=12 and k-point =1000
  V=120.59737 (a = 3.294) and E=-1707.621713Ry
   
  Then how can I do the optimization?
   
  Thanks in advance.
   
  With regards,
  Santhy Jaiker
   


Peter Blaha <pblaha at theochem.tuwien.ac.at> wrote:  Hi,

I hope it is clear that you CANNOT use min_lapw for your bcc Ti example.

With respect to the volume optimization, I don't see anything bad.

Specifying RKMAX does not fix RKMAX to this value, it just guarantees 
that it does not get bigger than what you specified, but when changing 
the lattice parameter, it is natural that RKMAX changes.

You shoulds add a few more negative volumes (-9 -12 maybe -15) and then 
plot the results because it seems the minimum is near -6. When you get a 
nice parabola it seems ok, if not it may happen because of too low RKMAX 
(gotro 7.5, 8, up to 9) or to few k-points (100 is very little).

> I am a new user of wien2k. I had gone through the FAQ and L.D. Marks 
> notes for optimization. I started with small system like Ti-BCC. 
> Before going to mini_lapw, I tried the accuracy test as same as given 
> in the site http://www.wien2k.at/events/ws2006/Exercises.pdf.
> 
> I gave lattice parameter= 3.294A°, RMT=2.5(default), RKMAX=7(default), 
> k-point=1000 and Gmax=12(default) for Ti-BCC system. I tried the same 
> step in the above site,
> STEP 1: run x optimize and generate 5 structures (-6, -3, 0, 3, 6 % 
> volume change)
> STEP2: edit “optimize.job”.
> STEP3: run optimize.job and plot the results. 
> I got
> SCF File RKM Energy
> Vol -6.0.scf *6.93 -1707.621717*
> Vol -3.0.scf 6.85 -1707.621662
> Vol 0.0.scf 6.79 -1707.621239
> Vol 3.0.scf 6.72 -1707.619926
> Vol 6.0.scf 6.65 -1707.618133
> 
> Energy is high when RKM is 6.93. Since I changed RKMAX=6.93 in Ti.in1 
> file and I didn’t change any other parameter. I repeated those 3 steps. 
> I got
> 
> SCF File RKM Energy
> Vol -6.0.scf *6.93 -1707.620994*
> Vol -3.0.scf 6.85 -1707.620941
> Vol 0.0.scf 6.79 -1707.620476
> Vol 3.0.scf 6.72 -1707.619422
> Vol 6.0.scf 6.65 -1707.617423
> 
> Here I observed that even though same RKMAX (=6.93) gives different 
> energy. And I couldn’t keep RKMAX as constant. Since, during SCF 
> calculation it changes.
> 
> Sorry I took more of your time. I have some experience to handle CASTEP 
> (in windows mode). But I couldn't correlate things in WIEN2k. 
> 
> It will be really helpful, if you tell, how to do the accuracy test for 
> RKMAX.
> 
> Thanks in advance.
> 
> Santhy Jaiker
> 
> 
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with love,
 K. Santhy
 
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