In the file CalcOmega.cpp generated by SARAH, there is the line “ForceUG = 1;”. It seems that that’s supposed to mean “force unitary gauge”, but where does that actually make a difference? Do we have to set ForceUG = 1 for micrOMEGAs model files generated by SARAH?

Unfortunately, I couldn’t find any mention of “ForceUG” in the documentation for micrOMEGAs or SARAH.

Yes, it's a command of micromegas to force unitary gauge. I think, it should be given in one of the several documentations. In general, unitary gauge is usually faster because of the reduced number of Feynman diagrams.