QuantumATK Forum
QuantumATK => General Questions and Answers => Topic started by: Mohammed on December 2, 2016, 13:38
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Hello,
I am running a DFT simualtion on anthracene molecules with a phosphonate anchor group. In order to get an agreement with experimental results I wanted to use the B3LYP but I understand its not included in the functional library in ATK. I used BLYP instead but I get a much smaller band gap (2.2eV sim compared to 3.5eV exp). I not familiar with all the functionals included but I am trying different sets to see if I can retain the correct band gap.
But I would greatful if you can guide me on how to get the correct value.
Thank You
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You may try using Meta-GGA as described in the following tutorial http://docs.quantumwise.com/tutorials/inas_2d_mgga/inas_2d_mgga.html.
I would like to note that, for semiconductor and insulator materials, the Kohn-Sham energy band gap that is actually calculated in density functional theory is not the same as fundamental energy gap, in general. So, Meta-GGA is one of a possible way to resolve this issue.
Using hybrid functionals would be another option, e.g., the HSE06 functional is available in the FHI-AIMS calculator, http://docs.quantumwise.com/tutorials/fhiaims_hse/fhiaims_hse.html, but you should keep in mind that hybrid functional calculations are much more expensive computationally, compared to the LDA, PBE and Meta-GGA functionals.
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I recommend to use the exchange correlation, Hybrid-B3LYP in the FHI-aims. And you should check your spin state. If you have unpaired spin on your molecules, you should set to the spin polarized, SHybrid-B3LYP. I hope you get a comparable result.
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Thank you for your replies.
I will try with the Meta-GGA functional and see the output. I understand that the band gap would be underestimated compared to experimental values but a difference of more than 1 eV seems quite large to me.
On a different note I wont be able to use the FHI-Aims calculator as it is not included in our license.