I am interested in polymers that contain "Benzathiadiazine"
Just structure or also thermochemistry?
For structures def2-SVP is usually okay, better def2-TZVP. For energies with DFT at least def2-TZVP, better def2-TZVPP or def2-QZVP.
In general if you need an answer fast without too much computation, its better to use composite methods (B97-3c, r2SCAN-3c) as these are by far more efficient and balanced than most standard DFT approaches.
https://www.basissetexchange.org/
Here you can see basis set and characterization basis of elements.
Jan-Michael Mewes thank you. I need to find theoretical energy differences between HOMO & LUMO, IR & NMR spectra.
Waseem Ahmad Ansari thank you for the reference.
You can perform opt+Freq and after completing open log file you can find HOMO LUMO ,IR and NMR perform with GIAO method.
Benzothiadizine isn't a big molecule, for that you actually use wavefunction methods like CCSD(T), maybe with a Dunning basis set. If you have actually big molecules that contain thadizine units, the advice by Jan-Michael Mewes is right.
Jürgen Weippert benzothiadiazine is not a big molecule, but it can be polymerise using several other molecules. I hope you got better understand about my question, & thank you for the answer.
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