Spectra of small molecule and computed spectra

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SUMMARY

The discussion focuses on calculating the spectra of a small molecule, specifically benzene with a nitrogen group, using Density Functional Theory (DFT) with the B3LYP functional and a triple-zeta valence (TVZ) basis set. Participants noted significant discrepancies between computed and experimental IR and Raman spectra, particularly due to the difference in environments (gas vs. water). Recommendations include considering larger basis sets to improve accuracy and understanding the expected deviations in calculated frequencies compared to experimental results.

PREREQUISITES
  • Understanding of Density Functional Theory (DFT)
  • Familiarity with B3LYP functional and TVZ basis set
  • Knowledge of vibrational spectroscopy techniques (IR and Raman)
  • Concept of solvent effects on molecular spectra
NEXT STEPS
  • Research the impact of basis set size on DFT calculations
  • Explore different DFT functionals for small molecule spectra
  • Learn about solvent effects in computational chemistry
  • Investigate methods for optimizing DFT calculations
USEFUL FOR

Chemists, computational chemists, and researchers involved in molecular spectroscopy and DFT calculations will benefit from this discussion.

greisen
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Hi,

I am calculating the spectra of a small molecule - benzen with nitrogen group attach. I have some experimental data of the molecule IR and raman.
I use DFT B3LYP with TVZ as basis set when I compare the modes and are a lot of differences between the two.

1. Would it help to increase the size of the basis sets?
2. How big a deviation should one expect?

The calculation is in gas whereas the experiment is in water - that should change the location of some of the modes.

Any comments appreciated thanks in advance
 
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I assume that you are comparing the calculated frequencies with the experimental ones? If so, it is very likely that the calculated frequencies are somewhat higher than the experimental ones. Is this the difference you are seeing?
 
yes it is this difference. Is there a way to determine if one is going to use DFT which functionals to use and the size of the basis set in order to optimise the calculation?
 

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