I ran a QST3 calculation under an expanded basis set:
S 0
6-311+G(2df,p)
****
H C N O 0
6-311+G(d,p)
****
(this is the same expanded basis set I used for optimizations)
When the calculation finished, the output provided a geometry that was nearly identical to my product but it contained 1 imaginary frequency at -255. I don't believe this to be the transition state. How can I refine the calculation to get a better result?
If you used GAMESS you can increas the Max SCF lteration and Max steps from Advanced-1
There is no simple solution of this problem. Try to improve your guess geometries in QST3 calculation. To get better guess and save time first optimize it in lower basis set.
If you are getting negative frequencies using DFT (atleast for the Minnesota suites or other meta-GGA), that don't always guarantee you a transition state. The imaginary frequencies can be a result of spurious oscillations in your ground state PES. These oscillations are present when your integration grid is not dense enough. Try using a more sophisticated grid and see if that makes the difference.
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