QuantumATK Forum
QuantumATK => General Questions and Answers => Topic started by: Mohammed on February 25, 2019, 15:32
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Hello All,
I would like a second opinion on some band gap analysis I carried out on 3 different types of Anthracene based SAMs. Their illustrations are attached ("SAMs.png"). I also investigated packed versions of the A2- & A3mono (i.e. with an overlap at the side chains). The band gap analysis was done both in standalone and on a substrate configurations ("Standalone.pdf" and "On Substrate.pdf"). One would expect with the increased conjugation, the band gap would decrease. And indeed this is the case either in gas phase or on substrate. But what puzzles me a bit, is the rather significant decrease in the gap in the case of the A3mono (either single or packed). To double check the MES anaylsis, I also did a PDOS simulaiton which returned more or less the same results
A second opinon on why the large decrease is observed would be greatly appreciated. Thank You!
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It would be helpful if you explain in more detail on why you are expecting narrowing the HOMO-LUMO gap in all the cases of the increased conjugation. That might be the key for understanding the issue. It could also be instructive to see your scripts for the "trouble" calculations.
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Apologies if I wasn't entirely clear. In terms of conjugation, I was refering to the single A1, A2, and A3mono. The addition of the lateral substituent groups (and their size) would lead to slightly smaller band gaps, which is rightly observed. This might not be the case comparing the "packed" configurations with their single counterparts, as there will be energy level splitting (this also has been observed for 2 ,3, and 4 adjacent molecules). I was interested to include the packed configuraitons in my analysis because when I move towards the "on substrate" anaylsis including boundary conditions, I try to mimic a thin film of SAMs (eventhough the nature of the overlap isn't exactly known!!).
On the substrate I am a bit puzzled by why the A3 massively decreased comparing it to the A2 and A1 (either in a single or packed configuration).
Regarding the script, I attached the one for the single A3 on substrate for the geometry optimization. Once finished I projected the MES only on the molecule