Editing Tips for (TS) optimization (section)

= TS search =
== Useful Keywords==
You will have notice that this is the hardest. Some keywords that helped me:

maxstep=X to see where the algorithm "lost" the TS, and restart with calcfc. Very slow method. 
Alternatively, ask me (cgoehry@iciq.es) my script to see in molden which atoms are concerned by the Eigenvalue-following TS search

vshift=X in case the optimization goes up and down very quickly (with metal atoms).

== Useful tricks==
The calculation of force constants (calcfc) is extremely demanding. So you can do it, or use one of the following ways around (use checkpoint files):

*Compute the force constant at a lower level, and use it at your favourite level. Ex:
      #p "method" freq=noraman  STO-3G
      --Link1--
      #p opt=(TS,readfc,noeigentest)  "method" 6-31G*

*Use the QST3 algorithm. Remember in Gaussian you have to put in order reactant, product and THEN TS. 

*A possibility if you want to "renew" your force constant set: 

when you find what a good guess for your ts, export it (ts.xyz), optimise normally with maxstep=10,maxcycles=10 or so, you'll go down (product.xyz), then "mirror" the change in coordinates. Ex:
      paste product.xyz ts.xyz > temp.xyz
      awk '{print $1,2*$2-$6,2*$3-$7,2*$4-$8}' temp.xyz > reactant.xyz

Then, use those structure within a QST3 scheme. This trick also works if the TS search "looses" your TS.

*One common problem in big molecules is the slowly convergence at final points of TS (when you know that it is your TS but the geometry oscillates around the saddle point). I usually restart 
the calculation using IOP(1/8=x) where x can go from 1 to 10 normally. It reduces the stepsize in the optimization (the default is 0.1 Bohr or Radian, and the modified is stepsize=0.01·x Bohr or Radian).
I usually select 4 or 5.