Struggling to learn abinit

jmullen · Post by **jmullen** » Wed Aug 18, 2010 7:42 pm

Greetings all,

I have been using Quantum Espresso for a few years now and I am atttempting to learn abinit through the tutorials as well as reproducing previous results from QE. So far, I have succeeded in all but one task, which is reproducing the band structure of graphene. I start with a configuration for the rprim, acell , and xangst for the atomic positions. I have calculated multiple datasets for changing acell to find the minimum energy and ionmov=3 for relaxing the atomic positions. The energy minimization curves look good in that I get well-fitted data, but when I try to run the band structure along high symmetry lines, I still get a band gap at K. Of course, I am sure this is due to my mistakes, so I am hoping that someone can point out where I have gone wrong. For brevity, I am attaching one input file which shows the calculation parameters for the band structure which are a result of the convergence studies.

Thanks for any help
Jeff Mullen
North Carolina State University

Robin · Post by **Robin** » Thu Aug 19, 2010 5:05 am

Dear Jeff,

The coordinates of K point is incorrect, which should be (0.577639, 0.577639, 0) in your system. By the way, an ecut of 120 Ha is too large for carbon; the comment for xangst is incorrect.

Sincerely,
Guangfu Luo

jmullen · Post by **jmullen** » Thu Aug 19, 2010 6:41 pm

Thanks for your response.

I replaced my k points in the input file as you suggested and the final energies calculated are

kpt= 0.5776 0.5776 0.0000 (reduced coord)
-14.42183 -13.59451 -8.77891 -2.58747 0.75599 9.20998 10.94101 11.02132

I am unsure where to go from here. Either my geometry is wrong, or I am calculating the wrong k point path (using the wrong coordinates).

Thanks for your help
Jeff Mullen
North Carolina State University

P.S. Attached are my new input file and the .files (named is changed because I could not upload the "files" extension)

Robin · Post by **Robin** » Fri Aug 20, 2010 4:39 am

Dear Jeff,

Check the files I use and the results they produce.

Sincerely,
Guangfu Luo

jmullen · Post by **jmullen** » Mon Aug 23, 2010 8:54 pm

Thanks for your help. Between your input file and several hours of experimentation I fixed the multiple problems I had with my geometry and am now reproducing the desired results.

Jeff Mullen
North Carolina State University

mverstra · Post by **mverstra** » Sat Sep 04, 2010 9:45 am

Hello,

in passing, you shouldn't need 120 Ha unless you are doing all electron carbon (note-pwscf uses Ry)

your atomic positions were probably not precise enough (use reduced coordinates 1/3 2/3 0 to make sure you preserve symmetry)

The K point, however, was correct as stated "1/3 1/3 0"

You could have problems because the default shift for k-points (0.5 0.5 0.5) has not been removed. You might check this and reset it with "shiftk 0 0 0"

Matthieu

jmullen · Post by **jmullen** » Wed Sep 08, 2010 8:26 pm

Hi Matthieu,

Thanks for your comments. Since making the original post, I have improved my understanding of the units and how to declare a unit to be in eV instead of Hartree when I need it as well as many other parameters. Once I realized these mistakes, I immediately calculated the correct results.

Regards
Jeff

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