[Wien] Would like to have some guidance from you

[Peter Blaha]

As a beginner, you need to gain experience.

So first start with pure A2BO4.  Does it run properly, check timings,...
Then create first a smaller supercell, eg. a 2x2x1 cell. Does it run ?
timings (4 times as many atoms, --> without iterative diag. ~ 64 times
as long cpu-time in lapw1. However, in the supercell you can use 4 times
less k-points, s the total effort does NOT increase that much.

For the large cell: use the   top  command to check the performance of
lapw1 or lapw2. In particular you should see of it uses the cpu by nearly
100 (or 200 when OMP_NUM_THREAD=2) %, or it does not becuase it does not
have enough memory and "pages".

Also check the timings listed in case.output1up/dn. Is the cpu-time and wall-time similar ?
If wall-time is much larger, your computer does not have enough memory.

k-points: You heard at the workshop about k-points: metal-insulator or small-large cells.
most likely I would not start out with so many k-points, but I don't know the details
of your system. Of course 50-kpoints will run 4 times faster than 200 k.

Another hint: For large cells you do not want to start out with RKMAX=7 (the default).
Start the calculation with RKMax=6 or even 5.5. Run to scf, it should be MUCH faster
(10-100 times !!!), but then don't stop, but increase RKMax and compare eg. the forces
on all atoms, magnetic moments, DOS, (... what ever you are interested in) to find a
reasonable RKMAX.

And finally: For these large cells, iterative diagonalization should be used.

I guess the most important message is: Start out with smaller problems. Experiment
with parameters like k-points, RKmax, -it to get experience.

Of course, a modern PC (or 4 of them for k-parallel runs) will help. But checkout the
real bottleneck (memory ?) before.

Am 07.07.2011 02:58, schrieb Dr Qi Wen YAO:
> Dear professor Blaha and Wien2k users,
>
> Thanks to the Wien2k Workshop, now that I can do some proper calculation but since i am new to Wien2k and I would like to have some guidance in my current calculation.
>
> I am doing a supercell (3x3x1) calculation (for publishing quality) to a layered structure perovskite A2BO4 compound. It is of K2NiF2 structure (SG I4/mmm), its resistivity is at around 0.001 Ohm-m at room temp so it is semi-conductive.
>
> Half of the "A" atoms (A atom is Sr in this case, and the B atom is Co) are replace by an Rare Earth atom so the whole supercell is of 90 atoms in size in my struct file. The original unit cell parameters were a=b=3.8 Amstrong and c=12.3 Amstrong.
>
> I am using a PC with 2 core Xeon 2.4GHz, 4G RAM running latest Susie, latest Wien2k and ifort11.
>
> The calculation is Spin Polarized (and not an anti-ferromagnetic as I know that this compound is ferromagnetic), I used 200 k-points for the current calculation.
>
> The initialization seems working fine (thanks to professor Blaha's guidance in the workshop).
>
> Now it seems that the calculation is a bit too much for this computer - it runs painfully slow and after two days, it only just displays to me two lines: "LAPW0 END" and "LAPW1 END".
>
> I am wondering: for this calculation, can I use less k-points say 50 (as it is a big cell) and the result would still be good enough for paper publishing?
>
> What kind of speed up do I expect if I am to use a 50 k-pints instead of 200 k-points?
>
> Would you think that a computer with i7 core and 16GB of RAM would be able to run this calculation without too much trouble (I am trying to convince my boss to purchase a new PC for me for WIEN2k)?
>
> Any comments/feedback would be greatly appreciated.
>
> Thank you for your time.
>
> kind regards,
> Qiwen
>
> **********************************************************
>
> Dr QiWen YAO
>
> JSPS Fellow
> Multifunctional Materials Group
> Optical and Electronic Materials Unit
> Environment and Energy Materials Research Division
>
> National Institute for Materials Science
>
> 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
> Phone: +81-29-851-3354, ext. no. 6482, Fax: +81-29-859-2501
>
> **********************************************************
>
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> Wien at zeus.theochem.tuwien.ac.at
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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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