I think of it in terms of the probability
of finding an electron within the volume enclosed by the electron density
map. In general, the probability of finding an electron close to
the nucleus will be higher than that of finding an electron far from the
nucleus due to electrostatic attraction between the nucleus and the electron.
If you're talking about a Gaussian distribution, the 1sigma, 2sigma,
and 3sigma levels correspond, respectively, to 68%, 95%, and 99.7% of the
population. As you've observed, an electron density map contoured
at 3sigma is very tightly contoured around the nuclei. This means
that there's a very high probability of finding electrons within the volume
of the electron density map, i.e. close to the nuclei. The same electron
density map contoured at 1sigma will be loosely contoured around the nuclei
since there's less probability of finding electrons farther away from the
nuclei. Good luck,
[ccp4bb] Meaning of sigma level of electron
density map?
I am new to the filed of crystallography. I am having
trouble figuring out what exactly does sigma level of electron density
map mean.
When sigma level of a map is increased (say from 1.5 sigma to 2 sigma)
why the map covering individual residues becomes less wide and more "precise"?
Shouldn't it be the other way round if they have anything to do with the
Gaussian districution?