Hello I am trying to build a 3D Poisson solver using method of moments. I need to find out the Green's function for the system. My system is a rectangular box and boundary conditions are as follows: On all surfaces BC is neumann. Only on the upper and lower surface, the middle 1/3 region has dirichlet BC. the other two 1/3 regions on both side of it are also neumann. I am really stuck on how to calculate the Green's function. The equation for Green's function is so simple (Del)2G = delta(r-r') But I can't incorporate the BCs. Please help!!!!