Open NMR: How good is the prediction?

Egon asks about the quality of the prediction:

Egon Willighagen Says:
November 5th, 2007 at 5:58 pm e
Peter/Henry, I was wondering about the carbon shifts for atoms in an aromatic ring? Can the QM method you are using work that out? Or can it not distinguish between C=C atoms and C:C atoms (SMILES notation)? The shift differences might really be too small… if I look up C6H6 and C6H8, they are indeed small.

PMR: Nick is the expert here and I'm waiting to talk with him. However here is the first aromatic structure on my list (nmrshiftdb2470-1 (solvent: chloroform)) – I haven't gone searching.

PMR:  You can see the agreement between obs and calc is good. The maximum deviation seems to be about 2 ppm. The aromatic region is quite disperse (the two substituents have widely different 13C shifts so that probably helps). The NMR confirms the assignment in NMRShiftDB. I would feel reasonably happy that the shift differences were large enough to identify the atoms. Obviously if the spread was smaller it would be harder. I'm waiting for Nick to collect all the data and mount it.
Note that this is an intra-molecule comparison and I haven't concerned myself with absolute differences. We believe there is a solvent effect on many carbonyls.  Henry is better placed to comment.

EW: How about 1H NMR prediction, or can QM not do that?

PMR: Yes it can, but the dispersion is much less. 13C is from 0-200, 1H is from 0-10. So an error of 2 ppm in C is not too bad, but in 1H it's a disaster. Also H atoms are more likely to be affected by conformation, shielding effects and solvent. So it may well be useful, but it will require a lot more work