GGA in Single electron Transitor

[Shan]

Hello all....

I have two questions related to Benzene Single Electron Transistor (SET) tutorial.

1)   In the tutorial LDA was used to perform the calculations, which is an inaccurate method.
      I tried GGA and MGGA with DZDP. MGGA not working. However GGA is working. why was GGA not used here?.

2)   In page-17, while calculating the MolecularEnergySpectrum for different charge states in SET environment, the Energy_Zero was kept to Absolute_Energy.
      Whereas in page-19, while performing the gate scan for different gate voltages using python script, the Energy_Zero parameter was kept to Fermi level.
      Why differently?
check out the manual at the link  http://quantumwise.com/documents/tutorials/latest/BenzeneSET/BenzeneSET.pdf

these basic things are confusing me a lot..... Please help me in this regard.

Thank u very much for reading the post...

and thank u in advance.

[Anders Blom]

1) Why should GGA be used, in your opinion? What is wrong with LDA?
2) The spectra saved from the script on page 19 are actually never used, so it doesn't really matter. I guess the idea was just to have them in the NC file in case someone is interested in looking at then. But for comparison I agree it would be more clear if the energy zero was the same as on page 17.

[Shan]

Reportedly, LDA underestimates the results...
and GGA is more accurate method than LDA.

For example in SET case Total Energy Calculations are crucial for differentiating the performance of SETs.
If the total energies were underestimated, it implies all the parameters that depend on total energies will also be underestimated.


Expecting your kind reply soon....
Thank you.

[Anders Blom]

Well, we just used LDA as an example, it's easy to change to GGA and rerun all the calculations. The results are not particularly realistic anyway, as the electric fields we apply are so large they will rip the molecule apart

[Shan]

Isn't it possible to make the structure realistic ??.

If not, what is the need of that SET tutorial?. only to understand some phenomenon ?.
It will be much useful if the structure can be made realistic....



Thank you Prof. Anders.
 

[Anders Blom]

Tutorials are not intended as full case studies, they are examples of HOW you can analyze things with ATK. A real study of a realistic system typically takes much longer time, whereas a tutorial should be possible to complete in a day or so. Not that GGA would have been slower really, but more importantly it would not have made any difference in the workflow. Also note that the results obtained for the affinity and ionization energies are actually fairly accurate with LDA. It's not 100% the case that GGA is always significantly better.