How to intelligently define parameters for "disp" in DFT calculation?


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I need to add dispersion correction to three range-separated functionals (CAM-B3LYP, LC-BLYP, and LC-WPBE). I see in the documentation that alpha and s6 need to be defined by the user, although defaults are available. My understanding is that there are default values available for any functional.

So, I guess my question is--how were the defaults chosen and is it wise to just stick with the default? If it is better to figure out alpha/s6 values on my own, do you have any recommendations of literature or other sources of information that would help me to pick appropriate values for a particular functional, basis set, and system?

Thanks in advance for any advice.