This stage in generating a preliminary model is determining the remaining undetermined euler angle for the symmetric class averages determined in the last step. This step is performed as part of startcsym or starticos.
The symmetric particles are stored in sym.img by the previous step. In the case of Cn (n-fold rotational) symmetry, the first class average is a top view, and the second class average is a side view. This statement specifies 2 of the 3 euler angles for the particle. However, we don't know how the side view is aligned with respect to the top view, so an unknown euler angle remains. This angle is determined using a common-lines method at this stage. In the case of icosahedral particles, the 3 class averages represent 5-fold, 3-fold and 2-fold rotational symmetries. In this case, there is 1 unknown euler angle for each particle.
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