Certain keywords have specific output control meanings, such as FOCK, VECTORS and DENSITY. If they are used, only the final arrays of the relevant type are printed. If DEBUG is supplied, then all arrays are printed. This is useful in debugging the subroutine ITER. DEBUG can also increase the amount of output produced when certain output keywords are used, e.g. COMPFG.

When DEBUG is present, unrecognized keywords are noted, but the job is not stopped.  There are many undocumented keywords that are useful in testing MOPAC.