From owner-chemistry@ccl.net Tue Sep 22 11:31:01 2015 From: "Grigoriy Zhurko reg_zhurko : chemcraftprog.com" To: CCL Subject: CCL: The use of NMR spectra computations for verification of computational method Message-Id: <-51765-150922111143-17822-vTcxVJlN6W//A84TbFkzeA*_*server.ccl.net> X-Original-From: Grigoriy Zhurko Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=us-ascii Date: Tue, 22 Sep 2015 18:11:54 +0400 MIME-Version: 1.0 Sent to CCL by: Grigoriy Zhurko [reg_zhurko||chemcraftprog.com] > 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. 1) So, what are these functionals and basis sets? 2) I still need to know, whether the B coefficient in Y=A+B*X equation of the correlation must be equal 1. > Frank > Frank Jensen > Assoc. Prof., Vice-Chair > Dept. of Chemistry > Aarhus University > http://old.chem.au.dk/~frj > -----Original Message----- >> From: owner-chemistry+frj==chem.au.dk::ccl.net [mailto:owner-chemistry+frj==chem.au.dk::ccl.net] On Behalf Of Grigoriy Zhurko reg_zhurko^chemcraftprog.com > Sent: 21. september 2015 19:20 > To: Frank Jensen > Subject: CCL: The use of NMR spectra computations for verification of computational method > 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.