Two dimensional surface, with a projected contour plot. It shows
two minima (M) and a reaction path (in red) connecting them.
The maximum in the red line represents the TS.
This plot can be used to illustrate a general computational feature. It is possible
to calculate the red path from the TS down hill, ending in either of
the two minima. This is what the IRC calculation in MOPAC does.
Such a path is also called an MEP, a minimum energy path.
In the TS it is clear that the calculated reaction coordinate should be
followed. Outside the TS, there are more negative values in the second
derivative matrix (updated after a certain number of steps), and
the algorithm has to select one (the largest negative?) to follow downwards.
The opposite action, calculating the path from M to TS, is not possible.
A gradual ascent from the minimum on the right would follow the blue line
rather than the red one.
('if you are somewhere in the mountains and knock over your bucket of ping pong
you can pretty well predict where they will go; but for the people
in the valley it is difficult to tell where the balls came from'.)
The MOPAC saddle algorithm (paragraph 2C) uses information from the
other side of the hill, the geometry on the other side of the TS, to 'pull' the
two geometries together.
Back to paragraph 1A.