Dear Firedrakers, I am trying to solve two coupled problems on different domains (essentially a small box next to a larger box), where the solution at the boundary from one problem determines the Neumann boundary condition for the second problem at the interface. I am not sure how to efficiently use the solution of the first problem to prescribe the boundary condition for the second. My idea was to derive from the Expression class and overwrite the eval function including a couple of if statements to make sure the coordinates are in the region where both meshes actually match. Is this the correct way to do it (it seems quite slow, probably because the derived expression can't be compiled in C) or is there a cleverer way to do this? Cheers, Daniel -- Dr. Daniel Ruprecht University Academic Fellow School of Mechanical Engineering, Room 4.50 University of Leeds Leeds LS2 9JT, UK Email: d.ruprecht@leeds.ac.uk Phone: +44 (0)113 343 22 01