[gpaw-users] Speeding up calculation for DFT/MD?

[Toma Susi]

Dear gpaw-users,


Background:
I am setting up a series of calculations to study the electron beam damage of carbon nanomaterials. To get at the physically relevant number, the established procedure is to give a target atom a starting momentum, and then run a MD simulation to see if the target atom escapes the structure, or is recaptured by the vacancy. This methodology has been employed to great effect by Kotakoski et al. using VASP. They also found that tight-binding (at least in the typical parametrization) only gives a correct description for all-carbon system. For system which I am interested in, DFT is needed.

Understandably, running a MD simulation with full DFT is rather heavy for the ~100 atom systems that are needed to ensure there are no spurious unit cell edge effects, and the up to 1000 timesteps of 0.1 fs required to give a sufficient trajectory. In VASP, choosing the parameter PREC = low or medium was used to speed up the calculations, with the justification that errors from the MD steps are anyway dominant.

I've established a 8x6 unit cell graphene with a grid spacing of 0.18 Å and 5x5x1 k-points as my reference system, ensuring full convergence in all characteristics I am interested in. However, running the MD simulation with this accuracy results a 6.5 minute SCF cycle time for each time step, when running with 128 cores on the new CSC supercluster Taito (http://datakeskus.csc.fi/en/superkoneet-ja-infra?param=param). Thus I would like to find ways to speed up the calculations without excessively compromising on accuracy. And yes, DFT really is needed for the systems I am interested in :)


On to the actual question:
Has anyone established GPAW parameters that would roughly correspond to the VASP setting for PREC = medium or low? I did preliminary checks with increasing the grid spacing, and did find that the SCF cycle time per time step decreased to about 4.25 min with a grid spacing of 0.25 Å. Have I understood correctly that a larger grid spacing than this is not recommended?

I tried also to play with the calc convergence{ } parameters, but didn't immediately find a combination that decreased the computational time (convergence seemed to be slower with most changes). Following the instruction to "write {'bands': -10} to converge all bands except the last 10. It is often hard to converge the last few bands in a calculation." in the Wiki really didn't help since although the density perhaps converged faster, the WFs didn't converge at all.

Any other tips / parameters to speed up the calculations? Since the density changes rather gradually between each small time step, could we somehow take advantage of this?


Many thanks in advance for any help you could offer,
Toma Susi

http://physics.aalto.fi/personnel/?id=322
http://mostlyphysics.wordpress.com

"Love is the only emotion that enhances our intelligence." 
	-Humberto Maturana

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