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