Hi everyone,

I would like to study the interfacial interaction effect of metal oxide by  diffrent (hkl ) plane
i calculat the interaction inergy between tow layers plymer / metal oxide by fllowing the tutorial in MS 
polymer interaction with a metal oxide surface

First an amorphous cell of polymer withe 8 repeat unit  was built, and then another layer of metal oxidate (with the same valeus of tutorial)was also built. 
After that I used the Build Layer tool to put them together, followed by Geometry Optimization and Molecular Dynamic (NVT), and then interaction energy was calculated.
my question is:
 how to increase the surface area and chang the periodicity of the metal surface Fe2O3(012) and (001) who is deffrent withe the metal in MS tutorials Al2O3(00-1)?? how i can increase the supercell and chang the vacuum thickness for my metal_oxide fe2O3 (012)(001) 
i don't know why  the polymer energy is positive valeu ??
 can  anyone guide me step by step to enswer my question 

Thanks in advance.