On Sat, Oct 8, 2011 at 11:27 PM, Dale Tronrud
"more precise than is actually the case"? I don't understand this. A map has precision to infinitely fine spacing. The sampling grids we choose are the artifacts - the courser the grid the worst the representation. The prismatic points and spikes of a coarsely sampled map are aliasing artifacts. A smoothly varying surface is an accurate representation of the continuous density function.
The spacing between grid points is telling you something about how well each of those grid points is resolved. Even if the electron density is continuous, it still comes from an incomplete Fourier series and is full of artifacts and ambiguity. Spacing your grid points every d_min/6 A implies (to my eyes, anyway) that the optical resolution allows you to accurately distinguish the values at those points, which isn't actually the case. It's not necessarily mathematically inaccurate, but since most of us are trained to model-build using a grid spacing of d_min/3 or d_min/4 (or whatever the default is in Coot), we "know" what a 3A map looks like, and a 1.5A map, etc. (I know this is all nit-picking, but I have in mind a specific figure in a methods paper where the authors compared a 2mFo-DFc map before and after their magical map improvement procedure, with much more detail visible in the "after" maps. I had to read it twice to realize that the "after" map had a much finer grid spacing - of course it looked much better!) -Nat