#!/usr/bin/tclsh

proc frametoframevec {sel {first 0} {second end} {size 0.2} {color red}} {

   set molid [$sel molid]

   $sel frame $first
   set firstpos [$sel get {x y z}]
   # note: no update, we want to follow the _same_ atoms
   $sel frame $second
   set secondpos [$sel get {x y z}]

   set gid [graphics $molid color $color]
   foreach f $firstpos s $secondpos {
      lappend gid [vmd_draw_arrow $molid $f $s radius $size]
   }
   return $gid
}