ionic convergence problem

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peterklaver
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ionic convergence problem

#1 Post by peterklaver » Wed Mar 08, 2006 1:44 pm

Hello people,

At present I'm doing lots of interstitial relaxations. A few configurations are giving me trouble with their ionic relaxation. At first the relaxation seems to go fine. But when there are just a few atoms left with force components of 0.1x eV/A the energy starts to oscillate. I've tried both IBRION=1,2 and I've also tried using very small POTIMs. Then I thought it might be the case that the forces are not accurate enough, althought the drift is much smaller than 0.1x. Anyway, I tried setting various precision-related prameters to finer values, but that didn't help. Seems logical, as the drift was small to begin with. I also tried more k-points but to no avail. The energy convergence of one attempt is given below. The starting configuration had already been relaxed a good deal. And the first 4 steps of this relaxation also go fine, but then things go wrong:

1 F= -.10638914E+04 E0= -.10640265E+04 d E =-.106389E+04 mag= 234.4195
2 F= -.10639015E+04 E0= -.10640369E+04 d E =-.100583E-01 mag= 234.4539
3 F= -.10639055E+04 E0= -.10640406E+04 d E =-.407408E-02 mag= 234.4406
4 F= -.10639093E+04 E0= -.10640441E+04 d E =-.376387E-02 mag= 234.4244
5 F= -.10639078E+04 E0= -.10640428E+04 d E =0.147456E-02 mag= 234.4241
6 F= -.10639068E+04 E0= -.10640418E+04 d E =0.105198E-02 mag= 234.4259
7 F= -.10638890E+04 E0= -.10640250E+04 d E =0.177652E-01 mag= 234.4554
8 F= -.10638818E+04 E0= -.10640182E+04 d E =0.715494E-02 mag= 234.4691
9 F= -.10638979E+04 E0= -.10640337E+04 d E =-.160660E-01 mag= 234.4537
10 F= -.10638989E+04 E0= -.10640348E+04 d E =-.100234E-02 mag= 234.4605
11 F= -.10638998E+04 E0= -.10640355E+04 d E =-.863053E-03 mag= 234.4618
12 F= -.10638980E+04 E0= -.10640339E+04 d E =0.176092E-02 mag= 234.4625
13 F= -.10638911E+04 E0= -.10640273E+04 d E =0.694348E-02 mag= 234.4661
14 F= -.10638833E+04 E0= -.10640198E+04 d E =0.776095E-02 mag= 234.4681

I've put input files and OSZICAR/OUTCAR files of my various attempts (9 MB) on

http://titus.phy.qub.ac.uk/group/Peter/files.tar.gz

Any suggestions as to what I'm missing here?

greets,
Peter
Last edited by peterklaver on Wed Mar 08, 2006 1:44 pm, edited 1 time in total.

peterklaver
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ionic convergence problem

#2 Post by peterklaver » Fri Mar 31, 2006 11:08 am

Solved it. The calculations which were giving me trouble had configurations where the system was very close to a saddle point. Understandably a difficult task for any ionic convergence algorithm. Starting from a configuration slightly further away from the saddle point helped it to converge properly. Apparently, the 'history' built up during the relaxation can make the crucial difference.
Last edited by peterklaver on Fri Mar 31, 2006 11:08 am, edited 1 time in total.

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