[Wien] optimization procedure - questions

Yevgen Melikhov Thu, 05 Feb 2015 06:11:57 -0800

Dear Prof. Blaha,
Dear users of WIEN2k,

I have several questions on how best to perform optimization procedure for the 
following problem:

I have a system with 96 atoms (it is relatively big in order to accommodate 1% 
of Mn in GaAs), which I refer to as System 1. The other system is the same but 
with two vacancies, so, overall, I have 94 atoms in this System 2. 

My first step is to relax both structures (assuming fixed lattice constant) 
before calculating X-ray absorption spectra.

Logically, I do not expect severe changes of atomic positions in System 1. 
However, for the System 2, I expect some severe rearrangements (to be confirmed 
yet). 

In the WIEN2k User Guide it is said that there are two methods to solve 
relaxation problems:
   (i) using min command, and 
   (ii) running run_lapw with MSR1a switch in case.inm file.

Am I correct to assume that for System 1 the method (ii) should work 
fine/faster? 

Am I correct to state that usage of the method (ii) for System 2 is wrong or at 
least will take much more time to optimize positions? In fact I tried using 
this method (ii) for System 2 and after 2,000 (!) iterations I gave up. I can 
see that some atoms, which are expected to move, do move. But their positions 
have not converged after so many iterations. 

As my system is magnetic, will it be correct to optimize first System 2 without 
spin polarization with method (i) and then fine tune the positions with spin 
polarization with method (ii). I think by doing it this way, I should speed up 
my calculations. I saw a discussion on this forum which took place some time 
ago with respect to magnetization and relaxation/optimization urging not to do 
that but, as magnetic atoms (Mn only) are quite far from each other this should 
not lead to a problem for my System, should it?

Finally, due to the nature of the LAPW method, could it be better to do 
optimization of atomic position using DFT software with plane waves and 
pseudopotentials and then continue calculations in WIEN2k? In the literature I 
found that some researchers do that but I wonder whether this is really that 
time efficient? 

Any comments will be highly appreciated.

Sincerely yours,
  Yevgen Melikhov
    Institute of Physics PAN, Warsaw.

P.S. I did try to read <Mixer_Readme> (the section “Parallel Atomic 
Minimization Algorithms (a.k.a. the Energizer Bunny)” specifically) and 
<Optimization Notes> by Prof. Marks.

P.P.S. When running the method (ii) for System 2 for 2,000 (!) iterations, I 
did not forget to decrease RMT by 10%. When I run command nn I did not see any 
two atoms being very close to each other.

Answers are at the following link:

https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg11810.html