Spectra of small molecule and computed spectra

greisen

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

chemisttree

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?

greisen

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?