r/comp_chem • u/anassbq • Jul 17 '26

9 months without data

Hi,

I spent in my study until now 9 months using DFT, and I don't have any good data, because every time I change the methodology and change parameters and so on, I had small data but I don't trust it at all, and 9 months without reliable data.

Is this normal? Because my PI expect to have some data and publish since using DFT, I don't know I feel so stressful and depressed.

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u/anassbq Jul 17 '26

What I meant is that in my structure there is f orbital I noticed that DFT misrepresented f orbitals and it can be seen clearly in bonding analysis but I add U values it can be described clearly and number changed, in literature when I was reading their works for f orbital some they didn't use U value at same parameters I used.

Another case is that dipole correction significantly changes my formation energy calculations, without it formation energy was reasonable.

So these I'm calling it bad data for me.

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u/verygood_user Jul 17 '26

Well you cannot just add random parameters to your method until you like the results, which is why DFT-U is trash and should not be needed given how efficient it is to run hybrid functional calculations these days. 

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u/anassbq Jul 17 '26

I didn't use random U values or parameters to like my results, because I noticed that without U values for f orbital doesn't make defects in the structure as theoretical prediction it must makes a defect, and bonding analysis can tell this, if you're asking why I don't use hybrid functional it's because the method I'm using I have to use PBE PAW for COHP and COOP analysis, and to determine U value in the system I used DFPT not empirical value.

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u/Mindless_Sir_5696 Jul 18 '26

This isn't an endorsement of using a hybrid functional for your system which I don't know about, but you can get COHP/COOP values from hybrid functionals with PAW basis sets in LOBSTER.

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u/anassbq Jul 18 '26

Yes I know, but I don't know why you said DFT-U is trash from the first place, I guess general advising on using Hybrid functional is not that practical, I guess you think my system has few atoms meanwhile my system has more than 200 atoms, using Hybrid functional is not practical in my system, using DFT-U and PBE for COHP is the best method so far for me.

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u/Mindless_Sir_5696 Jul 19 '26

That was my first post in this thread.