Thank you for the script.
To answer your questions:
- 1) There are no options for fixing the graphene interlayer-distance through the Script Generator. However, I think there is no reason to expect that the in-plane lattice constant of graphene will change when constructing the graphene dimer. Therefore, it should suffice to:
a) geometry optimize a single graphene layer (i.e. determine the equilibrium lattice constant of graphene).
b) fix that lattice constant while varying the graphene-graphene distance and determine the minimum-energy distance.
- 2) I have verified that fixing the unit cell Z-component during stress minimization does not work when using the DFT-D2 method. This is a bug and will be taken care of real soon. However, if using the method outlined above, this should really not worry you.
Two further comments:
- You really only need two carbon atoms in a hexagonal unit cell to represent graphene (Builder -> Add --> From database --> Graphene [try also Graphite]). This will reduce the number of carbon atoms from 36 to 4 for your dimer, and significantly speed up your calculations!
- For your information, ATK 2015 will implement the DFT-D3 method, the latest (and probably final) version of the Grimme dispersion corrections. Might be worth a try
