[gpaw-users] LCAO mode for Zeolite

[Marcin Dulak]

On 07/12/12 17:40, Ask Hjorth Larsen wrote:
> Dear Baldissin
>
> 2012/7/12 baldissin<baldissin at yahoo.it>:
>> Dear gpaw users
>>
>> I am calculating the site preference of Al substitution in zeolite.
>> Unit cells are massive ( 20x20x13 Ang ). My idea is using LCAO mode.
>> I am wondering if the method is accurate enough to discriminate energy
>> with difference of some meV (5-20).

>> any opinion?
>>
>> thanks a lot
>> Gael
>>
>> PS:   calc = GPAW( mode='lcao' ,  h=0.18, nbands=-5, xc='PBE',
>> txt='atoms.out', kpts=[(0, 0, 0)], usesymm=True , charge=-1,
>> maxiter=150   )
> Energies in LCAO mode are usually off quite significantly.  However
> trends are reproduced very well in my experience.  You may need to do
> it the hard way and compare to some grid-based calculations.
make sure you control the eggbox effect (it's of order of meV per atom), 
especially with many atoms per cell.
Here is how you can estimate the eggbox error of the total energy of an 
atom for different grid spacings:
python gpaw/tasks/eggbox.py -g "32:45:4" O -l -p 
"xc=PBE,mode=lcao,basis=dzp,convergence={'energy':0.00001}"
python gpaw/tasks/eggbox.py -g "32:45:4" O -s
Check also the periodic table at 
https://wiki.fysik.dtu.dk/gpaw/setups/setups.html

Consider also using the planewave mode (something like mode=PW(600)) for 
verification of the results.
See example 
https://trac.fysik.dtu.dk/projects/gpaw/browser/trunk/gpaw/test/pw/slab.py
You need the GPAW trunk 
(https://wiki.fysik.dtu.dk/gpaw/download.html#latest-development-release).

Marcin

>
> Be sure to follow the advice for optimizing the performance of the
> Poisson solver in such a large system:
> https://wiki.fysik.dtu.dk/gpaw/documentation/lcao/lcao.html#notes-on-performance
>
> (Don't reduce the solver's tolerance, but make sure the number of grid
> points is divisible with a high power of 2).
>
> You don't need to specify kpts and usesymm above.  You will need
> basis='dzp' set if you want sensible numbers though.
>
> Regards
> Ask
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Marcin Dulak
Technical University of Denmark
Department of Physics
Building 307, Room 229
DK-2800 Kongens Lyngby
Denmark
Tel.: (+45) 4525 3157
Fax.: (+45) 4593 2399
email: Marcin.Dulak at fysik.dtu.dk

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