Project Nucleus integrates the Nucleus simulation engine from Maya into Revit Architecture and Project Vasari. It allows designers to experiment with "form-finding" in the conceptual design phase by simulating forces directly in Revit Architecture and Project Vasari. (The latest technology preview of Project Vasari includes the Project Nucleus functionality.)
Project Nucleus can simulate a wide range of physical phenomena in real time, like wind, gravity, constraints, and collisions. These forces can help architects generate free-form shapes, many of which would be impossible to model by hand.
The technology preview ends on December 31, 2011. Any designs that you create remain valid after that date, but the Project Nucleus add-on will no longer operate when the technology preview terminates.