BackgroundI'm currently working with two quantum software packages: Gaussian 09 and CFour.  CFour does a very good job giving explicit control over how you want to invoke frozen core approximations and explains how the default handling is done via the Frozen_Core=On option.Gaussian 09 has a documentation page regarding frozen core.  All my Gaussian 09 jobs with frozen core use the FC flag in the route section.  It is important to note that frozen core is basis-set dependent, meaning, the criteria for choosing which core(s) to freeze is directly correlated with the implemented basis set.I've implemented the following basis sets in Gaussian:aug-cc-pVDZ aug-cc-pVTZaug-cc-pVQZaug-cc-pV5Zaug-cc-pV6Zaug-cc-pV(T+d)Zaug-cc-pV(Q+d)ZWhat I NeedI need to know if CFour and Gaussian 09 handles the frozen core approximation the same (particularly related to the listed basis sets above).  More specifically, if I use the FC option in Gaussian, will invoking the FROZEN_CORE=ON option be sufficient for comparable data or will I need to use the DROPMO command and explicitly tell CFour which core(s) to freeze?  If the latter, how do I determine which orbitals to freeze as to match that of Gaussian 09?The answer may be right in front of me on the documentation pages but it seems rather ambiguous.  Of course some benchmark computations could be done to analyze this but I'd rather ask before embarking on such an endeavor that eats precious CPU clock cycles.EDIT:This question is strictly about program handling and job control.
How does Gaussian 09 handle 'Frozen Core Approximations' and how does it compare to CFour handling?
computational chemistry
The canonical way to freeze cores as described on the Gaussian keyword page is to freeze all non-valence molecular orbitals (MOs) as determined by SCF. This means that for $n$ core electrons the first $n/2$ MOs in ascending MO energy are frozen. This applies only for post-SCF methods. If you want to ensure that CFOUR and Gaussian09 freeze the same MOs you should probably specify explicitly which MOs to freeze as per the ListWindow keyword in Gaussian.