Thanks for your reply. I have already changed the autoshutoffmin to 1e-7. What I noticed that this (very quick simulation within ~ 1-2 mins) is happening when I used separate Mesh constraint for my simulation. In fact my simulation region is large and I am using 3D FDTD for that (because varFDTD is not giving proper result since the structure has different cross-sections). Any suggestion regarding how to reduce simulation time in 3D FDTD for large structure is highly appreciated. Thanks.nn