just a comment to Sam's comment. Using CIS for DFT does make sense and several programs also allow to do that. Then one usually speaks of TDA-DFT, where TDA stands for the Tamm-Dancoff approximation. From another point of view, if you take the RPA equation (TD-HF) and evoke TDA, then the resulting equation is the CIS equation. As linear response TD-DFT is very similar to the TD-HF equations, you can do the same for TD-DFT and get TDA-DFT and I assume that is what Massimo wanted to do when he tried to evoke CIS/B3LYP.

However, I am not sure whether Gaussian allows it. I know that NWCHEM and ORCA allow to do TDA-DFT. Excitation energies are very similar to those of full linear response TD-DFT, at a lower computational cost. Transition dipole moments were reported to be more effected by this approximation, however.

I hope this could help.

Cheers,

Lars

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Dr. Lars Goerigk

School of Chemistry (Building F11)

The University of Sydney, NSW 2006

Australia

Profile: http://www.researchgate.net/profile/Lars_Goerigk

List of publications: www.researcherid.com/rid/D-3717-2009

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Dr. Lars Goerigk

School of Chemistry (Building F11)

The University of Sydney, NSW 2006

Australia

Profile: http://www.researchgate.net/profile/Lars_Goerigk

List of publications: www.researcherid.com/rid/D-3717-2009