Asking for help; I can not relax a ZnO surface!

[hadi9827]

Dear all,

I am trying to relax ZnO (10-10),(11-20), and (0001) slabs by using the semiempirical method, Slater-Koster. For (10-10) and (11-20) the calculations are fast and the surfaces are stable and in a good form and everything is reasonable. However, for the (0001) slab, I tried several slabs with different sizes and boundary conditions but the calculations will converge in a very long time (2-3 days!!!) while the surface finally collapses or deforms. I attached the script and a pic. Would you please guide me on what the problem could be and what the solution is? I have been struggling with this for one week, trying any solution I assumed may work 
Thanks.

Hadi

[hadi9827]

The problem has been solved. The 0001 ZnO surface is not stable and needs some modifications to become stable. Just to share what I found out: if anyone is interested in more info, he/she may take a look at the following paper:
http://dx.doi.org/10.1016/j.susc.2012.09.004

Thanks,
Hadi