ZGNR-BNNR structure and optimization

[MaryamNazirfakhr]

Hello

I use ATK2013. I'm trying to build a unitcell of a 8ZGNR-4BNNR hybrid nanoribbon.
1. Once I used the "merge" tool, in order to merge 8ZGNR and 4BNNR, and then I optimized the whole 8ZGNR-4BNNR unitcell.
2. I created a 12ZGNR, then I changed 2 dimer lines of C atoms to B and N atoms (4BNNR), finally I optimized the whole   8ZGNR-4BNNR unitcell.

I have 3 questions:
1. I would like to know that which one of the mentioned methods is  more recommended?
2. what is the lattice constant for a ZGNR-BNNR structure? (when i used the first method, the lattice constant was 2.505, and by the second method it was 2.46. )
3. I use DFT for optimization. I set cut-off and k-point sampling to 150Ryd and 5,5,5 respectively. the optimization setting is the defeult setting. is it correct? what is your recommendation for the setting of scripter?

Thank you

[neerajkumar]

If you are dealing with nanoribbons then your k-point sampling is not correct. You should select k points which give  periodicity along that of ribbon and only 1 point in rests of the two directions.

I think first method is more appropriate since in second method you might miss the global minima of the system. To find correct lattice constant, you can use total energy minimization method i.e. plotting total energy as a function of lattice constant.

Best Wishes