A saddle point on the PES should show only a single negative vibration when doing a frequency analysis. This negative vibration should correspond to the process your transition state corresponds to. For example, when you would like to find the TS of a bond-breaking process, the negative vibration should be a vibration in which this bond stretches.
If you find multiple negative vibrations, your geometry is not a saddle point geometry yet. You should therefore do a RUNTYP=SADPOINT calculation, hereby reading in the calculated Hessian (the product of the frequency analysis which can be found in the PUNCH file) with "$STATPT HESS=READ $END".
When optimizing your geometry to a saddle point, Firefly will try to keep the strongest negative vibration negative while making all other negative vibrations positive. If the negative vibration you would like to keep negative (i.e., the one that corresponds to your TS) is >not< the strongest negative vibration in your frequency analysis, you can tell Firefly which vibration to keep negative with the IFOLOW keyword (in the $STATPT group). For example, if the second most strongest negative vibration is what you would expect for your TS, specify IFOLOW=2. Note that, in my experience, this doesn't always work well.
If the above fails or if you don't see the vibration corresponding to your TS, then the geometry you provided is most likely not close enough to the TS structure. In such a case you need to provide a better starting geometry, i.e. one that is closer to the TS geometry.
The Firefly manual has a section dedicated to geometry searches. Please have a look at it, it contains lots of useful information.
Hope this helps.
On Mon Feb 20 '12 11:30pm, martha barajas wrote
>so if I understood well, even if until now I have found several negative frequencies, it is possible to perform the sadpoint calculation using the Hessian that I have, then I can find in the output file a single imaginary frequency?
>because I thought it was no longer necessary sadpont calculation, since this structure that I proposed,showed me that have several negative frequencies.
[ This message was edited on Tue Feb 21 '12 at 2:48pm by the author ]