[gpaw-users] FD vs LCAO vs PW
Jens Jørgen Mortensen
jensj at fysik.dtu.dk
Tue Jul 1 09:44:48 CEST 2014
On 07/01/2014 09:03 AM, Michael Walter wrote:
> Dear Ask,
>
> would it be possible to put your nice summary to the gpaw-web pages ?
> This could be useful for others and such thinks are not easy to find
> in email conversations.
After seeing Roberts email yesterday, I started to write something for
the web-page:
###################################################
The following table can guide you in choosing the right mode, but you
better run some tests yourself also.
====================== ===== =========== =========
FD LCAO PW
====================== ===== =========== =========
memory consumption large small medium
speed for small system slow fast fast
speed for large system fast very fast slow
Absolute convergence easy complicated very easy
====================== ===== =========== =========
With LCAO, it can be hard to reach the complete basis-set limit and get
absolute
convergence of energies, whereas with FD and PW mode it is quite easy to do
by decreasing the grid spacing or increasing the plane-wave cutoff energy.
##################################################
But seeing Ask's brilliant reply, I also think that it would be valuable
to have that on the web-page.
Ask: Should I commit my piece and then you can add to it?
Jens Jørgen
>
> Best,
> Michael
>
>
> 2014-06-30 21:53 GMT+02:00 Ask Hjorth Larsen <asklarsen at gmail.com
> <mailto:asklarsen at gmail.com>>:
>
> Hi Robert!
>
> Generally:
>
> LCAO has the advantage over FD/PW that it's fast for larger
> systems since you have fewer degrees of freedom in the
> diagonalization.
>
> FD/PW have the advantage over LCAO that it's easy to crank up the
> precision by setting grid spacing or planewave cutoff.
>
> PW has the advantage over FD that planewaves are "nice" (they tend
> to better represent wavefunctions) and you need less to have a
> reasonable representation. Also the planewaves don't suffer
> egg-box effect like grid points do. Apparently stuff like
> response calculations are much nicer in PW mode.
>
> FD has the advantage over PW that you can throw many CPUs at
> domain decomposition. In PW, you can only parallelize over
> k-points/spins/bands. If you want do do many hundreds of atoms,
> PW would therefore be less useful.
>
> PW is extremely efficient for small-ish periodic systems.
>
> In GPAW specifically:
>
> PW has the slight disadvantage that it's new-ish and thus more
> prone to things not working.
>
> I think PW is still the only mode that implements the stress tensor.
>
> LCAO has the slight disadvantage that it's quite different from
> the two other modes, and e.g. some advanced functionals have not
> been implemented with LCAO.
>
> LCAO is very inefficient for systems where the cell is smaller
> than the basis functions.
>
> Did I leave anything out? Any questions?
>
> Best regards
> Ask
>
>
>
> 2014-06-30 17:48 GMT+02:00 Robert Warmbier
> <Robert.Warmbier at wits.ac.za <mailto:Robert.Warmbier at wits.ac.za>>:
>
> Dear GPAW users and developers,
>
> I was wondering if someone could provide a comparative pro/con
> list for the different modes GPAW supports. There is very
> little information on the GPAW website, as far as I could find.
> If there is already information and I missed it, I am sorry.
>
> Best Robert
>
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> --
> ------------------------------------------
> PD Dr Michael Walter
> Addresses:
> - Fraunhofer IWM, Wöhlerstrasse 11, D-79108 Freiburg i. Br., Germany
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