# DOS calculation with TBTrans

I calculated the Transmission curves and the Density of States using TBTrans. In particular, if I have pristine graphene (just for the sake of the example), I obtain as Transmission curve and DOS respectively:

How come the DOS calculated by TBTrans has these peaks while the Transmission looks perfectly fine?

• The general rule on the site is to have 1 question per post. If you have related follow up questions, you can ask them in a separate post and link to this question for any background/context.
– Tyberius
Jan 22, 2023 at 20:56
• Ok, sorry! Since this is already bountied, I will wait for an answer. If I won't receive any, I will make two separate posts. Jan 23, 2023 at 9:33
• @Tyberius from the revision history you can see that I commented out the question that was labeled as #3. Based on your comment, do you think I should have done that for #2 as well? I was thinking about doing that too. I value your opinion on this. Laura: the fact that theres a bounty, means that it's even better to have the question focused on just one thing. If there's two sub-questions, and users know the answer to sub-question A but not to sub-question B, then they might not write an answer, and then your \$50 bounty is wasted. Jan 24, 2023 at 19:20
• I modified my question and open a new one for the second answer Jan 24, 2023 at 19:35
• @NikeDattani I was mainly commenting in an attempt to explain your edit, but I didn't make it clear that I was referring to prior to the edit, rather than before. I think the previous edit was fine, but it probably will be for the best to have each question in a separate post.
– Tyberius
Jan 24, 2023 at 19:50

First, it is not clear which DOS you are referring too? Is it Green function, spectral function or bulk DOS? In this case, try and plot them all!

There are several aspects that determine the smoothness of the DOS and transmission plots:

1. The $$\eta$$ value that determines the imaginary part of the Green function, both for the device part and the electrode part, a larger value will smooth out the DOS, at the cost of precision similarly to the electronic temperature in the fermi-dirac distribution. You should play with both $$\eta$$ values to see how it influences the DOS/transmission
2. The k-point resolution, you'll generally see some van-Hove singularities in the pristine graphene case, again, if you increase the k-point sampling you'll reduce the peaks and smoothen out the functions.
3. The transmission is generally less spiky as it involves both incoming and outgoing states, so the numerical influences of the $$\eta$$ becomes more apparent.

You should play with the $$\eta$$ and k-point sampling values to see how they are all connected.

• This was the DOS from the Green function. When I plot the DOS for the bulk graphene, I obtain exactly the same graph as in the Transmission at V=0 as expected. Since I would like to use the DOS for comparison with the transmission, and I think I can be satisfied with the plot that I obtain for the latter, would it be correct at this point to keep all the parameters the same and compare the transmission with he bulk DOS? Jan 25, 2023 at 9:39
• That is up to you :) I don't know why you want to compare and why, generally the bulk DOS and the device DOS are not the same. In this case they should be the same, provided that the parameters are the same (see my answer). So I think you should figure out which parameter that controls this deviation. Jan 25, 2023 at 10:13
• Reading the manual, I see that the eta values should be TBT.Elecs.Eta for the electrodes and TBT.Contours.Eta for the device. Should one play with both of them to check their influence? Jan 25, 2023 at 10:51
• We might need a Siesta chat room, it seems it would be busier than the quantum ESPRESSO one (chat.stackexchange.com/rooms/117642/quantum-espresso). Jan 25, 2023 at 11:09
• @Laura comments are "temporary" post-it notes and when they get deleted they never come back, so please try to ask new questions in new posts! Jan 26, 2023 at 15:52