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 ΔHf 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.