Dipole correction for slabs

[giacomo giorgi]

Dear all,
I have few doubts about slab dipole correction.
I am treating a 001 slab of anatase TiO2. I have simulated the H2O dissociative adsorption on the surface, i.e., -H saturating O at the surface and -OH saturating Ti.

Now, I saturated both the upper and the bottom layer. But only the 3 external upper layers are optimized, while the remaining two (and the H and OH saturating the bottom layer) are kept in the bulk positions.

Must I switch on LDIPOL=T and IDIPOL=3?
I mean, since the PARTIAL optimization, should I consider the system as an ASYMMETRIC slab?

The second doubt is: If I take the same, clean 001 unsaturated surface (no water adsorbed at all) and I want to simulate a molecular adsorption on top of it, say 1 molecule of CO (still keeping frozen the two bottom layers, and free to optimize the three upper layer), should I consider this slab as asymmetric one and thus correct the dipole?

In other words, what is the definition of an asym slab? Does it depend by the terminations of the slab (I think about ZnO (0001) that is my prototype of asymmetric slab) or also by the total/partial optimization of the layers? What about the "asymmetry" induced by the absorption of molecules ONLY on one face of the slab?

Thanks in advance
Very best!
G

[admin]

whenever a system with polar adsorbatesor dissimilar atoms in the 2 slab surface layers does not have a mirror plane in the central plane of the slab you should use the dipole corrections, it will improve the convergence of your calculations. you can also do a fast check: please do one run to generate a LOCPOT file (LVTOT=.True.) and check the behaviour of the planar-averaged potential in the vacuum (vtotav tool). If the potential shift in the vacuum between the 2 surfaces of the slab does not have a slope of 0 you should consider to apply dipole corrections.
[ Edited Thu Aug 26 2010, 04:52PM ]