An exact definition of the SCF test used in MOPAC is not easy to write because of the large number of qualifying criteria used, so in the following description some details have been left out. For most systems, the SCF criterion can be described as follows:

First, two quantities need to be defined

SCFCRT: This is the primary quantity used in deciding if a self-consistent field
exists. For most operations (`1SCF`,
geometry optimization, reaction path, etc.) the value of SCFCRT is 10^{-4}, but
for gradient minimization and `FORCE` calculations a
higher precision, 10^{-7}, is needed. For other calculations, the
precision can be increased to 10^{-6} using
`PRECISE`. The value of SCFCRT can also be set using
`SCFCRT` or modified using `RELSCF`. The value
of SCFCRT is a constant in any calculation.

SELCON: For all "difficult" calculations (`FORCE`, gradient minimization, RHF open
shell, etc.) SELCON is set to SCFCRT. For normal ground state
calculations SELCON is set to 100 times SCFCRT and possibly higher depending on
the gradient. The value of SELCON can vary from one SCF calculation to the
next.

For an SCF field to exist, the following test must be passed twice
consecutively: The change in ΔH_{f} on going from one iteration to the
next must be less than SELCON kcal mol^{-1}, and the largest change in density matrix element
values must be less than 0.05(SELCON)^{½}.