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General procedure for locating transition state structure.

1. Choose the starting structure (guest structure): The guest structure can be obtained from several ways. One way to this is to perform relaxed PES scan. It normally helps to understand the PES of the reaction pathway and can give a good starting structure for TS searching.

2. Run frequncy analysis: Frequency analysis should be performed on every guest structure. Most of the time, many negative frequencies will be observed at this stage but in some cases this could revealved that the guest structure is ,in fact, the TS structure (having 1 negative structure) !! Additionally, the calculated force constant from this frequency analysis can be further used in the optimisation of the TS.

3. Optimisation for transition state: If the frequency analysis revealed more than 1 negative freuqncies, Gaussian has a command designed for locating the transition state. This can be requested by using command opt=TS. Locating TS can be difficult, additional commands “calcfc” and “noeigentest” are normally used in TS locating jobs. opt=(calcfc,ts,noeigentest)

4. Test the genueinity of the TS: TS structure is confirmed by frequency analysis. The genueine TS must have only 1 imaginary (negative) frequnecy.

5. IRC calculation: IRC calculation is mormally performed to confirmed if the obtained TS connects the reactant and the product of interested.

How to fix when IRC path tend to goes the same direction irrespective of forward or reverse command

1. increase the stepsize slightly (default is 10) so try 15

2. use the local quadratic approximation algorithm (lqa)

3. still a problem use calcall, this is expensive it requires a frequency calculation at EVERY point

4. proton transfer, using cartesian coordinates helps

5. nudge the IRC by manually modifying the TS structure very slightly along the reaction vector in the desired direction (use minimally!!)

opt=(calcfc,ts,noeigentest) irc=(rcfc,stepsize=5,lqa,maxpoints=100,maxcyc=40,forward)