Free will, rationality and intelligence are inseparable in definition.
In FREE WILL-EVEN FOR ROBOTS, McCarthy gives a definition of free will which boils down to the ability to say “I can, but I won’t”. To be able to say “I can”, the system has to have built a representation of the world, complete with counterfactuals representing the way the individual parts of the world link and react to each other. To be able to say “I won’t”, the system needs to have a preference, that is, in its actions it is using its understanding of its cans and cannots (the counterfactuals) to drive the world towards a state more preferable to itself.
Seen this way, free will is a design pattern / framework. It is any representation-building goal-seeking system. Rationality, then, is a statement about the quality of the representations – if a free will makes choices that most effectively seek its goals, it is considered rational. Note that you need to know both the actions and the goals to determine rationality. Whenever I see papers on irrational behavior, I look carefully to see what goals they assume.
The study of human rationality poses problems because we often don’t know what people want, and it isn’t certain that you get the right answer by asking them. However, even when the goals sought are not known, there is progress that can be made towards assessing the presence of rationality. For example, if I assume that a person is walking with the goal of going from point A to point B, I don’t have to know what those points are to observe that any path with a U-turn is suboptimal. (I think this underlies the unwillingness to make U-turns even when they are optimal going forward, because they provide everyone around you with an undeniable proof of suboptimality.) I would be wrong to conclude this for a sight-seeing tourist though, or an oil tanker that gets diverted because it receives news that the price of crude is now higher elsewhere.
What of intelligence then? Well, counterfactuals are built by processing data from the senses / memory. I consider all quality difference attributable to the processing, and not the data, to be intelligence. This is often described in terms of speed – by locking a person up in a room, the time needed to reach the final conclusion cannot be due to new data, and must therefore be due to the processing, i.e. how fast or slow the person is. This is for cases where a final conclusion exists – where given enough time all people arrive at the same answer. In cases where the answers are persistently different, it is more difficult to examine intelligence by itself – I believe this is why the slow/fast terminology persists.