Editing Tips for (TS) optimization (section)

= Internal Coordinates =
Keywords "nosymm" and "cartesian" may help you to avoid the horrible "error in internal coordinates"

Also, you can go on cesca and try version b, c, d, which apparently defines internal coordinates slightly differently. 

If you use keyword "cartesian", set "maxcycles=100" because at 101 it starts going crazy for no reason. Then, wait and restart the job.

If you need "cartesian" but also a constrained optimization, a useful tip is to add a -1 in front of frozen atoms:

H -1 0.000 0.000 0.000