Cell optimization : z direction constraint

[ashish dabral]

Hi,

Can someone explain to me how I can do a Cell optimization with a fixed z 
direction(height) while everything else (including atomic coordinates)
can change? I usually take 20A as the z height (supercell z vector length) 
for my 2d systems to isolate image cell interactions but apparently 
some structures are contracting in the z direction and reaching bulk state 
after 200 iterations, which is really time consuming. I want the system
to move to prevent any stress but I do not want contraction because it does 
not remain 2d anymore.

Also, If I fix the vertical height, will it be right? I mean if the 
original system with that vertical height want to contract then it has 
interactions
with the image cell. If I fix the height then I would still have the 
interactions with the image cell above but will not see it in optimized 
structure.
Will this be right approach?


Thanks
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