Author Topic: HOMO/LUMO separation in heterocycles with 2 heteroatoms  (Read 136 times)

Oldmessaya

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HOMO/LUMO separation in heterocycles with 2 heteroatoms
« on: August 13, 2018, 12:46:51 pm »
Hi,

I want to make geometry optimizations of quinoxaline. But running the extended Hueckel I get the warning, that the HOMO/LUMO seperation is lower than 0.05 AU and by looking at the orbital energies I can see, that the "bigger energy gap" is between LUMO and LUMO+1. 

My boss told me that this is a well-known error in this kind of molecules, but we have no idea how to solve this problem to get a proper calculation.

If someone knows how to get the correct occupation, please tell me.

Greetings
 Oldmessaya

antti_karttunen

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Re: HOMO/LUMO separation in heterocycles with 2 heteroatoms
« Reply #1 on: August 14, 2018, 12:02:45 pm »
Hi,

As the extended Hückel is just an initial guess, there is not need to worry about the gap at this point. You can proceed and carry out a single-point energy calculation with ridft (assuming you want to use DFT). After this, you can check the orbital energies with command "eiger". If everything is OK, you can proceed with the optimization. If you have gaps in the occupation (negative HOMO-LUMO gap), you could use the keyword $fermi to change the occupation.

Best,
Antti

Oldmessaya

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Re: HOMO/LUMO separation in heterocycles with 2 heteroatoms
« Reply #2 on: August 14, 2018, 01:54:01 pm »
Hi again,

I want to perform an ricc2 optimization.

So I could perform a single-point calculation before the optimization to check the correct occupations?

Thanks for the reply.

Best,
Oldmessaya

antti_karttunen

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Re: HOMO/LUMO separation in heterocycles with 2 heteroatoms
« Reply #3 on: August 15, 2018, 11:15:28 am »
Hi,

yes, you could run a single-point HF calculation with dscf to check the orbital occupation. When everything is OK, you could submit the ricc2 optimization.

Best,
Antti

uwe

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Re: HOMO/LUMO separation in heterocycles with 2 heteroatoms
« Reply #4 on: August 15, 2018, 06:15:42 pm »
Hi,

@Antti: Thanks!

@Oldmessaya: Just a hint if you are new to Turbomole: Use the graphical user interface TmoleX. It will run all steps automatically in the right order. If the final occupation results in hole, the graphical user interface will show that in the Results panel (prints the HOMO-LUMO gap in red, showing a warning).

Regards,

Uwe