When INVERT is present, all optimization flags are reversed. That is, if an optimization flag is "1", it will be set to "0", and if it is "0", it will be set to "1"
Refinement of a transition state for a large system runs faster when only the atoms near to the transition state are used in gradient minimization, and the rest are used in energy minimization. INVERT is useful for calculations of this type. The optimization flags for all atoms in the gradient minimization are set to "1", the rest are set to "0". In one calculation, a normal gradient minimization is run, in a second calculation a geometry optimization is run, with keyword INVERT present.
The two calculations can be combined into one job. For example, the first data set, for the gradient minimization, can be followed by a second data set consisting of keywords "OLDGEO" and "INVERT" plus any other keywords needed for the job.
Alternatively, make two complete data sets, one for gradient minimization and one for energy minimization. Run these data sets on different computers. Then edit the resulting ARC files to make up a new data set. This saves time, because the jobs run twice as fast.