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SEGMENT-GROUP POSITIONING

    Activating the 'Segment-group Position' item of the 3D 'Edit' pulldown
menu allows one to interactively define a group of segments 
which may then be translated and rotated as a rigid body relative
to a Cartesian frame located at the group's center of mass or at the
backbone atom (C1') of a user-specified nucleotide. 
 
   The purpose of this type of editing is for taking care of cases not
handled by the 'Segment Position' or 'Stem-group Position' editing
functions.  For example, it is sometimes necessary to try changing the
position of a stem region.  This is not possible with the 'Stem-group
Positioning' function because any stem within the chosen group will be
moved as a whole, that is, all of its regions constitute one rigid body.
Nor is it possible with the 'Segment Position' function because it
requires two segments to specify a stem region.

   Needless to say, the 'Segment-group Position' editing feature is only
for achieving alternate starting conformations.  As in the unedited case,
these conformations should be regarded as first-order approximations
to stable ones, and hence the need for subsequent refinement.

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