Specifically I want to extend work involving B3LYP started with Gaussian 03 but continued with GAMESS-US. The energies provided by the default B3LYP methods are not the same. There is a discussion about this in the GAMESS-US manual (Further Information section):
Note that B3LYP in GAMESS is based in part on the VWN5 electron gas correlation functional. Since there are five formulae with two possible parameterizations mentioned in the VWN paper about local correlation, other programs may use other choices, and therefore generate different B3LYP energies. For example, NWChem's manual says it uses the "VWN 1 functional with RPA parameters as opposed to the prescribed Monte Carlo parameters" as its default. Should you wish to use this VWN1 formula in a B3LYP hybrid, simply choose "DFTTYP=B3LYP1".
It says the defaults are different between GAMESS and NWCHEM and that there is an option to get GAMESS to do the same calculation as NWCHEM is doing by default.
How do I get a G03 and a GAMESS B3LYP calculation to agree?
What are the differences between various software packages' default implementation of B3LYP and their capabilities, i.e. can their B3LYP definitions/implementations be adjusted?