I'm calculating HOMO and LUMO distribution for some molecules and getting strange results. All of them are anions at pH 7.4, so I'm using base set 6-311+G** and this is what I get for LUMO orbital. Is this normal and how can I interpret that? Or should I change my basis set?
Dear Marcos,
first of all, for anionic species it would be recommended to use the basis set with some extra diffuse functions, for example, 6-311++G**. And next, what software do you use for the visualization and what is the orbital contour value used for showing orbitals? I'm afraid that the contour value might be too large in this case, I would recommend to use something like 0.01 or 0.005.
Best regards,
Aleksey.
Dear Marcos,
If you are using Molden visualization software and contour value is set to what Dr. Kuznetsov is suggesting and still you see similar plot then try to use a basis set with more diffuse functions (by adding one more + or adding more diffuse functions manually to heavy atoms). If you get same results even after doing so, then this result seems to suggest that it is a Rydberg type orbital. Which common in heteraromatic systems for example, pyrrole.
Regards,
Avdhoot
Dear Avdhoot and Aleksey,
I'm using Spartan Wavefunction'10. For more details, I used the HF method.Do you think it is really a visualization problem with contour? I thought it was really strange, because it is the first time I see an orbital like that and besides the negative charge of thise molecule is far away from this orbital.
About the Rydberg type orbital, are they common or should I check for errors on my structure? I'm asking because all my molecules have this same ring and as I said, only those with negative charges showed this orbital.
Best regards,
Marcos
Dear Marcos,
it MIGHT be the visualization problem. But, also, it could be indeed the high-lying, or Rydberg, type orbital, especially because you have the negative charges on your species. Btw, what are the energies of your HOMO and LUMO? Also, I'm afraid that HF method here might be not very suitable. I will recommend trying DFT with larger basis sets.
Best regards,
Aleksey.
Mr. Santana,
As you have explained the computational steps the results look like nornal.
My gut always says to use Post-HF theories as well as DFT for something that seems quite fiddly such as this, might be worth doing to check if you get the same result. It takes a lot more power, but sometimes worth the effort.
Just another doubt... Is DFT, in general, better than HF for calculations like eletrostatic potential map, HOMO/LUMO distribution and energies?
And Thank you for the answers, it really helped me.
Energy wise, remember that they compute the energy in a different mathematical form to HF, so it honestly can depend on the complexity of your system. Though for HOMO/LUMO you should be fine with DFT, and your basis set is pretty large as well.
Dear Marcos,
strictly speaking, for vizualization purposes it's better to calculate orbitals using HF approach, but usually, as Emma said (and as my own experience says :-)) DFT works totally fine here.
Best regards,
Aleksey.
This is an absolute must read for all interested parties: https://pubs.rsc.org/en/content/articlelanding/2013/cp/c3cp52547c#!divAbstract
Your LUMO computed at HF looks unbound. What is the energy? This is not all that uncommon for HF. I'd use DFT with a very large basis set (of TZ quality or better) with diffuse and polarization functions.
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