Which one would be more suitable, LDA or GGA?

[lknife]

Dear all,

I am trying to do some optimization calculation on my layered TMDs. However, when I tried different "Exchange correlation", say, GGA and LDA, there were much differences between the results. The most difference is the interlayer distance.

For example, for my MoS2/WSe2 hetero-bilayer system, the optimized interlayer was 6.1723 Angstrom for LDA-OMX-medium-200 (here 200 is the density mesh cut-off ) and 6.728 Angstrom for GGA-SG15-high-75 (here 75 is the density mesh cut-off, too), respectively. Let alone some other differences such as the lattice vectors and the band structures. There was so much significant differences that I can hardly believe in both of them.

Which one would be more suitable for the optimization of these layered structures? and why? Can anybody kindly help me?

Thanks a lot to anyone who views this message!
 

[Petr Khomyakov]

None of the local (LDA) and semi-local (GGA) density functionals account for the van der Waals interaction that determines separation distance between the monolayers in layered materials. Please  have a look at the following tutorial on how the van der Waals interaction correction can be taken into account in the ATK-DFT calculations, see http://docs.quantumwise.com/tutorials/dispersion_corrections_and_bsse/dispersion_corrections_and_bsse.html 

[lknife]

Thank you very much! There are so much things I need to learn about ATK!