geometry optimization of Na slab

[Hongliu]

Dear Matt,

your suggestion helps indeed! With GAPW the cell optimization of a 6x6x6 
cell finished successfully with the bcc symmetry being kept quite well. The 
obtained lattice constant is about 4.20 Angstrom, close to that obtained by 
using other methods, for stance 
https://wiki.fysik.dtu.dk/gpaw/setups/bulk_tests.html.

For the geometry optimization of  Na-111-slabs the 3-fold symmetry is also 
kept quite well. Great! Thanks for the insightful hints.

But some new issues come out now:

For a 3-layer slab, the inter-layer distance changes from 1.2 Angstrom to 
0.8 Angstrom. This seems fine, more or less.

For a 4-layer slab, the two inner layers merge and the inter-layer distance 
of the resultant 3-layer slab becomes 1.9 Angstrom. Strange?!

For a 6-layer slab, the inter-layer distances are 1.15, 1.0, 1.55, 1.0, 
1.15, i.e. a wider gap appears between the upper 3-layer and the lower 
3-layer.

The bottom layer atoms are fixed. The input files and results for the 
4-layer case are attached.

Could you and other experts comment on that? Thanks!

Best wishes,
Hongliu
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