When users want to examine the results of a single SCF calculation of a geometry, 1SCF should be used. 1SCF can be used in conjunction with RESTART, in which case a single SCF calculation will be done, and the results printed.
When 1SCF is used on its own (that is, RESTART is not also used), then derivatives will only be calculated if GRAD is also specified.
1SCF is helpful in a learning situation. MOPAC normally performs many SCF calculations, and in order to minimize output when following the working of the SCF calculation, 1SCF is very useful. See An SCF Calculation for a worked example of an SCF.
When calculating the energies required to form electronic excited states, to avoid geometry reorganization, 1SCF must be used (see Frank Condon Considerations). This allows the vertical transition energies to be calculated (the Franck-Condon transitions).