SUMMARY: flat potential surface

Hi, All
 A couple weeks ago I asked the following question:
 Dear ALL,
 If the potential surface happens to be flat at the
 transition state region, what is the best way to find
 the 1st transition state. I tried to use conventional
 optimization method in G98 and it always produced an
 imaginary frequency which is not along the reaction
 I had a few kind responses and tried some of the
 suggestions. Unfortunately neither of them is working
 so far. Maybe it's because dft cannot handle the
 diradical species very well as seen in other examples.
 Here is the summary I got from some folks. Thanks
 again for all your suggestions!
 1) use qst2 or qst3
 2) use opt=calcfc
 3) use scf=tight, opt=(calcfc,tight)
 4) use int=(grid=ultrafine) scf=novaracc if using dft
 5) scan the co-ordinate and use two points on either
 size of the maximum as the starting poitn for
 6) if calcuating a loose complex or A+B->C then order
 the atoms carefully to keep more tightly bonded atoms
 on both sides next to one another
 7) go back to opt=(ts,calcfc) if qst2/qst3 completely
 near the ts.
 ... if after this, the surface is very flat, and the
 surface differs from the analytical surface, it may be
 practically impossible to find a well-converged
 1st order transition state using g98. If you find a
 way to
 handle this case, I'd like to hear about it.
 hope this helps a little,
 Jeremy Greenwood
 jeremy.greenwood-: at
 Department of Medicinal Chemistry
 bh +45 35306117
 Royal Danish School of Pharmacy
 fx +45 35306040
 Universitetsparken 2, DK-2100 Copenhagen, Denmark
 ah +45 32598030
             On the other hand, the early worm gets
    Men det kaern der kommer foerst til moellen, bliver
 foerst malet.
 Dear Eric,
 Which method are you using:DFT or HF/post-HF?
 If you are using DFT I am afraid it might not be so
 happy with the diradicals.
 Secondly, if it is a radical reaction, I am really
 suspicious of the presence of a pre- or post- reaction
 Also with QST2 and QST3 one should  be very careful
 while matching the atoms.
 Have you tried the options noeigen and nrscale whith
 opt? opt=(ts,noeigen,nrscale,calcfc) is my favorite
 for TS calculations.
 Sometimes Basis set superposition error can give lower
 TS than they actually are. There is a keyword
 'massage' for that but I think this is an extreme
 You can lower the convergence threshold but that will
 cost you some loss in accuracy. Or maybe you can
 optimize the geometry with lower convergence and then
 take this output and use it as an input for further
 optimization with higher convergence.
 You can also 'freeze' the reaction coordinate and
 optimize the rest of the molecule. Then you can
 'relax'the reaction coordinate too for further
 I hope you get the TS you are looking for soon..
 Sincerely yours
 Alimet Ozen
 ozensem-: at
 Dear Eric,
 First of all it depends on your reaction I think.
 There might be a complex located below GS and TS
 structures on PES.
 Also how about tunnelling, if there is a proton
 transfer, for example, in your reaction?
 You might try Opt=QST2 option in Gaussian, once it
 gave me very good results.
 Also you can optimize the TS first using a
 semi-empirical method in Spartan or Mopac or anything
 and then use the output geometry as an input in
 gaussian calculation.
 Sincerely yours
 Alimet Ozen
 ozensem-: at
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