Hi Boaz, That approach works too. Getting the exact threshold isn't really so hard using my method though. If using menu entry Measure Mean, SD, RMS reports RMS as 5.3 and I want the contour level to be 2 sigma, then I just type in 10.6 (= 2 * 5.3) in the volume dialog threshold level entry field and press the Enter key. Tom
Hi William,
The way I go about the sigma level contouring in Chimera, though it's somewhat ham-fisted but perhaps less complicated than Tom's suggestion, is to contour the region of interest at the desired sigma level in Coot or O and then play with the contouring in Chimera until both maps look as similar as possible (the maps rarely look identical for technical differences in the contouring techniques, I think). It works.
Cheers,
Boaz