HIya, im not actually sure why this is happening so was wondering if anyone can help. im running in photon mode and im trying to generate a weight window. initially i think my splitting mesh was over done a little bit (ok too much say about 15 fine intervals in a 10cm region) and i wouldn't go past one history. just billions of collisions were occurring. I reduced the number of splitting and now im getting histories but why did this happen? my geometry is fine but is very complicated. now that im getting histories, and scoring in some of the regions i want but im not getting a wwgout file. i do, however, get a warning saying some of my particles have escaped the wwg mesh. thanks in advanced. i am relatively new to mcnp but im on a mission to learn this!