2D: From MOPAC to Gamess

If a transition state geometry is found and characterized by MOPAC, it can be transferred to Gamess for more sophisticated energy calculations.
The keyword AIGOUT will produce a z-matrix in Gaussian format, which can easily be transformed to a Gamess type of input file.
See the .arc file obtained in paragraph 2B, for a Diels-Alder TS.
The basic layout is shown in an example in chapter 1.

Calculations can be performed using this geometry, but optionally it can be refined further, using the RUNTYPE SADDLE directive in the input file. (This corresponds to the TS algorithm in MOPAC, not the MOPAC SADDLE mentioned in paragraph 2C.)

Actually, the Diels-Alder reaction is not a good example for this type of calculations. It has been shown that elaborate methods have to be used to reproduce the synchronous mechanism. It is likely that a simple SADDLE calculation in Gamess, using the MOPAC TS geometry, will fail to optimize at a 'synchronous' saddle point. So this part is likely to change in the future.

However, AM1 calculations have been used to explain regioselectivity in Diels-Alder reactions. For a 'modern' virtual reality approach, see a contribution by Suñer, Casher and Rzepa to the ECTOC.

Back to chapter 2, How to locate a Transition State.