I'm trying to do g09 simulations to obtain raman spectra of nanographene flakes (~ 24 carbon atoms) doped with nitrogen. Can you suggest a theory / basis set to use? Are diffuse orbitals essential? Polarisation needed? Cheers in advance! p.s. I have used B3LYP 6-31G (d,p) so far, and my structures are 'exploding' around dopants. The graphene structure is ok.
I have no particular experience with graphenes but I would suggest trying out dispersion-corrected functionals such as wB97xD or B97D3 in addition to B3LYP, and using at least a triple-zeta basis set such as cc-pVTZ. Hope it helps :)
For graphenes, dispersion corrected functionals would work better than B3LYP.
If you want to use correlation corrected triple zeta basis set, consider using cc-pCVTZ instead of cc-pVTZ . If using HF+MP2 instead of DFT, you may consider the latter basis set with core orbitals frozen during post-HF computations. It is not good idea to use cc-pVXZ (X=D,T,5) if you cannot freeze the core orbitals, because these basis sets are not optimized for core orbitals.
I am also trying to do 2d graphene simulations in Gaussian 09 using PBEPBE functional. However for 6-31G(d,p) and 6-311G(d,p) I am getting a negative frequency. Surely, this is not correct. Can anyone suggest me what should be an appropriate basis set in this case?
You may use basis sets not included in Gaussian 09 with the ExtraBasis and Gen keywords. You can get the basis set you need already formatted for Gaussian from the Basis Set Exchange website: https://bse.pnl.gov/bse/portal
You definitely need a dispersion-corrected functional like B97D3, wB97xD or M06-2X. Since your system in not very large you can use at least a triple-zeta basis set.
good luck
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