CCL: The use of NMR spectra computations for verification of computational
- From: Frank Jensen <frj^chem.au.dk>
- Subject: CCL: The use of NMR spectra computations for verification
of computational method
- Date: Tue, 22 Sep 2015 06:13:15 +0000
Sent to CCL by: Frank Jensen [frj * chem.au.dk]
One cannot help wondering why people still are using B3LYP/6-31G* as the answer
to any question.
For NMR shielding, there are better functionals and basis sets optimized
specifically for NMR.
Using such a combination will have essentially the same computational cost, and
who knows, may even solve problems like the one posed.
Assoc. Prof., Vice-Chair
Dept. of Chemistry
> From: owner-chemistry+frj==chem.au.dk a ccl.net [mailto:owner-chemistry+frj==chem.au.dk a ccl.net] On Behalf Of Grigoriy
Sent: 21. september 2015 19:20
To: Frank Jensen
Subject: CCL: The use of NMR spectra computations for verification of
Sent to CCL by: Grigoriy Zhurko [reg_zhurko^chemcraftprog.com] I have been
computing NMR spectra of several organic compounds, comparing them with the
experiment. Usually these computations show big systematic errors, but correlate
well with the experiment (for C13 spectra of organic molecules, I got the
correlation coefficient R about 0.9995 with B3LYP/6-311G(D,P) method).
Now I have computed some NMR spectra of bilirubin molecule with different DFT
functionals, and I found that the correlation coefficient is not a good criteria
of computation accuracy in my case. This molecule has internal hydrogen bonds,
and different functionals (in particular, B3LYP and PBE) give quite different
O..H distance (the difference is about 0.1 A), while other bond lengths in this
molecule do not differ significantly (the difference is 0.012 A or less). The
PMR spectrum with B3LYP correlates with the experimental one with R=0.997, and
with PBE ? R=0.995. These values do not differ very much. But the coefficient B
in the equation Y=A+B*X (for the linear approximation of experiment vs theory
graph) is 1.02 for B3LYP, and 1.14 for PBE. So, with PBE it is far less from 1.
Does that mean that PBE is much less appropriate method for this task?
I suppose, that the systematic error of absolute values of the NMR chemical
shifts is caused by unclear physical meaning of these chemical shifts and
shieldings (maybe the solvent gives some additional shielding in experiment).
So, my question is, whether the B coefficient in correlation must be always
equal 1. If yes, then instead of correlation coefficients R I should use another
criteria of computation accuracy ? the RMS of MAE difference between the
computed and experimental chemical shifts, if the shielding of the standard
(TMS) simply fitted for best agreement (not computed quantum-chemically). Is
that correct? In my case, these MAE difference must be much bigger for PBE
functional, than for the B3LYP functional.