Editing G09 (section)

== The RI-DFT method ==
From the Gaussian manual:

Gaussian 09 provides the density fitting approximation for '''pure''' DFT calculations. This approach expands the density in a set of atom-centered functions when computing the Coulomb interaction instead of computing all of the two-electron integrals. It provides significant performance gains for pure DFT calculations on medium sized systems too small to take advantage of the linear scaling algorithms without a significant degradation in the accuracy of predicted structures, relative energies and molecular properties. Gaussian 09 can generate an appropriate fitting basis automatically from the AO basis, or you may select one of the built-in fitting sets.


What all this means is that for all calculations that have more than about 1000 basis functions, it will speed up the calculation greatly if you use the RI-DFT method. Generally, the greater the number of basis functions, the better it compares to normal DFT. The accuracy of this approximation is very good, and generally you can afford to use a much higher basis set than you would with a normal DFT calcualtion (so you get rid of BSSE errors etc.).


''' How to set up an RI-DFT calculation.'''

You have to use a pure DFT functional... no hybrids allowed... 

You also have to specify '''TWO''' basis sets... The first one is the normal basis set specification (GENECP is supported for complicated setups with transition metals and ECPs), then the second one is the "density fitting basis", which you can specify yourself or Gaussian can generate automatically.

For example:

 # PBEPBE/TZVP/TZVPFit

would specify a calculation with the PBEPBE functional, TZVP "normal" basis set, and TZVPFit "density fitting" basis set. The / sign between each section is essential.

For an example of an input file, and more information on how to set up the calculation, see the following page... [[RI-DFT input]]