Hi, P K, 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,
Hidong P K <[EMAIL PROTECTED]> Sent by: CCP4 bulletin board <[email protected]> 07/25/2008 12:38 PM Please respond to P K <[EMAIL PROTECTED]> To [email protected] cc Subject [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? Thanks!
