Chemistry - PBE vs. PBEPBE functional
Solution 1:
The designation PBEPBE is an actual terrible artefact from researchers using Gaussian. The actual publication[1, 2] only refers to it as PBE, and most other program packages (I know) implement it as such. Often they make the correlation and exchange parts separately available via PBEC and PBEX, or similar. Unfortunately this designation also made it into the literature.
The confusion further continues since Gaussian uses the PBE1PBE designation for the (standalone) hybrid functional, which is in most other packages known as PBE0.
But it doesn't stop there, for TPSS you may find similar naming/ reference schemes. The BP86 functional keyword requests the VWN(III) version, while many other programs use VWN(V) for this. The latter is available via BVP86, but misses any implementation of dispersion. We could go on with this...
In the end what is important is the correct referencing of the methodology so that the results are reproducible, i.e. look at the literature cited. Gaussian actually does a good job referencing those sources. And when you publish, you'd do us all a favour and not use the keyword designations of Gaussian.
[1] John P. Perdew, Kieron Burke, and Matthias Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865. DOI: 10.1103/PhysRevLett.77.3865. Mirrored at Burke's page: dft.uci.edu/publications.php
[2] (Erratum) John P. Perdew, Kieron Burke, Matthias Ernzerhof, Phys. Rev. Lett. 1997, 78, 1396. DOI: 10.1103/PhysRevLett.78.1396
Solution 2:
If you want to use a LDA or GGA functional in Gaussian you always need specify the desired Exchange and Correlation functional. In case of PBE you need to specify it twice, only using PBE will result in an error.
This is also explained in the online manual: http://gaussian.com/dft/ (Tab "Keyword: Pure Functionals")
Solution 3:
I came across this question by chance, and even if you already solved the problem, I wanted to add my contribution because of the many reads it keeps receiving. Actually, that notation (which is common mostly in Gaussian) implies that you are specifying both the exchange and the correlation part of the functional. It assumes that you write the functional as exchangecorrelation altogether, i.e. PBEPBE uses PBE exchange and PBE correlation. Following the same schemes, PBE1PBE (popular because of Gaussian, but it actually makes sense too) stands for "1 parameter hybrid" using PBE exchange and PBE correlation (it is the same as PBE0, as originally defined by Adamo and Barone). B3PW91, the first 3-parameter hybrid proposed by Becke stands for "3-parameter hybrid" using B88 exchange and PW91 correlation. Many other functionals are defined using this scheme. I hope this helps following readers!