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.