how to optimize a supercell under strain?

[wot19920302]

Dear Quantumstaffs:
         I want to optimize graphene nanosheet supercell under strain along z direction. This is my method to achieve it:
         1.  optimize a supercell to the  equilibrium state
         2. change lattice constant along z direction, here I use strech cell tool with "streched" instead of "moved" to apply strain, I think "streched" means keeping fractional coordinates constant and "moved" means  keeping certersian coordinates constant.
        3. when optimze the strained supercell, we should keep lattice constant along z direction constrained  and along y direction free, x direction is vacuum so it doesn't matter wether constrianed or free.
        This is my thought, am I correct?
         Best
     

[Anders Blom]

For constant strain, yes. If instead you want constant stress, you can use the approach described in http://docs.quantumwise.com/tutorials/uniaxial_biaxial_strain/uniaxial_biaxial_strain.html