By default, MOPAC uses the CODATA fundamental physical constants. This means that the results of MOPAC 6 and earlier MOPAC calculations cannot be duplicated. When OLDFPC is specified, the fundamental physical constants used in earlier MOPACs is used. This allows these earlier calculations to be duplicated using the current MOPAC.

Users are advised to not use OLDFPC except when comparison or continuity with earlier MOPACs is necessary, as all future MOPACs will use the new CODATA constants.