The result of an unconstrained geometry optimization is a stationary point on the potential energy surface. This point has several very important properties. Among these are:

- The geometry refers to a specific system, e.g., the starting point for a reaction step or a salt bridge or a neutral system.
- Its heat of formation can be related to heats of formation of similar
systems (by "similar" is meant systems with exactly the same formula and
charge, and calculated using the same conditions, i.e.,
`PM7`and`EPS=78.4`).

Because of the importance of stationary points, the corresponding ARC files should be properly labeled and stored for future use.

Also, because these structures are so important, unconstrained optimization
of protein systems should be carried out with unusual care. Use solvation
for all systems of this type, i.e. use `EPS=78.4`. Optimize the geometry. This might take CPU days
or even weeks of
effort, so to be on the safe side, use `T=4W`
`DUMP=2D`. If the optimization is not exhaustive subsequent geometric
operations might compromise any results - changes in heat of formation would be a
mixture of the original structure still changing and thus its energy decreasing,
and the energy change caused by the current geometric operation.

A useful
strategy is to optimize the geometry, then if the gradient norm in the ARC file is large, over
about 0.2 kcal mol^{-1} Å^{-1} per atom, simply
re-optimize the geometry. To do this, either run the ARC file without any
editing, or edit the ARC file to make a new MOPAC data-set, then run that.