Grouping procedure

What are groups for?

If you're going to group micrographs together it is important to understand what groups are used for inside RELION. Although you may be familiar with the concept of defocus groups in other packages, RELION groups are NOT the same. Each particle has its own (possibly astigmatic) CTF model, and groups do not in any way affect the CTF models. Instead, for all particles inside each group, RELION will estimate an average power spectrum for the noise (rlnSigma2Noise), as well as an average intensity scale factor. RELION will warn against using small numbers of particles inside a group, because these averages may become unstable. That in turn may lead to crashed runs that report errors like the sum of weights for certain particles being zero, or the scale factor being a strange number.