During a MOZYME run, the steady accumulation of small errors (for example, roundoff, and more important, the elimination of atoms from LMOs) will result in the LMOs becoming non-orthogonal. These errors can be eliminated almost completely by re-orthogonalizing the LMOs. If REORTH is present, then the LMOs will be re-orthogonalized at the end of every 10th SCF calculation, starting with the first SCF.

In practice, a better strategy is to allow the job to run for ~500 cycles or 1-2 CPU days, then shut it down and restart it using RESTART, but without using keyword OLDENS.  That will cause the LMO's to be recalculated, and job to resume starting with the geometry at the end of the previous run.