So how do we get close to a saddlepoint?

 

We can guess, using chemical knowledge and intuition (as for eclipsed ethane above). 

 

OR, for example:

 

Coordinate driving (also called adiabatic mapping)- optimize the structure of the system with a particular coordinate fixed:

·   A sequence of optimizations is performed with the coordinate fixed at a given value for each optimization, then changed to a different fixed value for the next optimization. 

·   This can generate an energy profile, with the approximate TS at the highest point. 

·   For example, for ethane, optimizations could be carried out with the value of one H-C-C-H torsion angle changed in a series of steps from 60° to -60°. 

·   The energy would be a maximum for the structure optimized with the H-C-C-H angle equal to 0°, which would be the model for the TS structure. 

 

Next: how to guess a transition state structure (continued)