## This takes a selection and moves that selection for every frame in the
## molecule so that the center of the selection is fixed (all atoms are 
## moved, but the move is based on the selection).
## 
## For example:  moveframes top "protein"
## 
proc moveframes { molid seltext } {
  set ref [atomselect $molid $seltext frame 0]
  set vr [measure center $ref]
  set sel [atomselect $molid $seltext]
  set all [atomselect $molid all]
  set n [molinfo $molid get numframes]
   
  for { set i 1 } { $i < $n } { incr i } {
    $sel frame $i
    $all frame $i
    $all moveby [vecsub $vr [measure center $sel]]
  }
  return
}