For barriers to rotation, inversion, or other simple reaction that does not involve making or breaking bonds
Optimize the starting geometry.
Optimize the final geometry.
Identify the coordinate that corresponds to the reaction. This is likely to be
an interatomic distance, an angle or a dihedral.
Starting with the higher energy geometry, use a path option to drive the reaction in the direction of the other geometry. Use about 20 points, and go about half way to the other geometry--the transition state is likely to be between the higher energy geometry and the half-way point.
From the output, locate the highest energy point--this will be near to the transition state.
Starting with the geometry of the highest energy point, repeat the path calculation. Use smaller steps (0.1 times the previous step is usually
good), and again do 20 points.
Inspect the reaction gradient. It should drop as the transition state is approached. If it does, then use TS to refine the transition state.
For bond making or bond breaking reactions involving exactly one change in
covalent bonding
Identify the reaction coordinate (the bond that makes or breaks)
Use a path calculation to drive the reaction.
The geometry of the highest point on the reaction path should then be used to start a TS calculation.
For barriers involving an angle or dihedral and no covalent bonds are made or
broken
Identify the angle or dihedral that would be used as the "reaction path"
Use a path calculation to drive the reaction.
The geometry of the highest point(s) on the reaction path should then be used to start a TS calculation.