I am doing QST3 calculations using Gaussian 6.0.16 to get the transition state structure for C-Cl bond fission in Propargyl Chloride (CH2CCHCl). since the reactant is singlet and products (C3H3 and Cl) are doublet, I have a spin contamination problem. what is the best way of solving this problem? I've tried using restricted open shell DFT-B3lyp/ 6-31G but I keep getting this error "No analytic 2nd derivatives for this method"
does anyone knows how to fix that?
Hello Seyedeh,
I wouldn't use QST3 for this system but do two scans: a relaxed (singlet) PES scan by increasing the C-Cl bond form 1.5 A or less to 3 or 4 A, and a relaxed (triplet) PES scan. The TS for this system (if exists) is most likely to be located at the intersection of these two scans.
Hi Seyedeh,
As Ahmad suggested, it is easier to use relaxed scan by gradually stretching the C-----Cl bond to obtain the TS. QST3 (or QST2) is helpful when you dont have clear idea how does the TS looks. Your question about the spin contamination isn't very clear. However, there are different ways to fix the spin contamination problems. If you run MP4 calculation, it, by default, does the spin-annihilation for the contribution from higher spin state and gives you the spin pure energy. If you want to stick with DFT, Yamaguchi spin-projection works in most of the cases. You can look at my paper (Article Theoretical Study of Oxidation of Guanine by Singlet Oxygen ...
) where I had explored spin-contamination on TS and some diradicals. Paper from Prof. H. Bernhard Schlegel on spin-contamination can be useful.Best,
Bishnu Thapa
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