Computing Transition States in ONIOM

This documents list the method I use for finding TSs with ONIOM.

1. Run a scan over the TS coordinate.<br\> If you get any discontinuities instead of a smooth curve around where the TS should be then you are scanning along the wrong coordinate Look for atoms that move a large amount in the discontinuous step and scan along this one instead.

2. Take the step that has the highest energy and the lowest RMS force (the most TS like structure) and optimize this with the TS coordinate frozen. Use geom=modredundant and add the specification under the molecule geometries.

3. Numerical compute the Hessian around the TS coordinate using geom=modredundant and specifying it under the coordinates. Use Opt=(ts,noeigen), also things to consider are the step size and the use of quadmac which seems to be pretty much required

So basically it is the same as finding a normal TS but your options are a bit more restricted (e.g. no QST2).