[yt-users] re-calculate function

Hi, Simpler question (I think!) Can I force yt to re-calculate a derived field? I'd like to call dd["Radius"] muliple times, but changing the position of the center field parameter in the same script. Currently, yt knows it's already calculated dd["Radius"] once and doesn't repeat it, even though "center" has changed. Ideally, I'd like to clear just that field, so yt doesn't go and recalculate any other fields. Elizabeth