# load trajectory
mol new nacl-water-1.gro waitfor all
mol addfile test.trr waitfor all

##############################
# define function to (re-)draw the dipole vector
proc do_boxdraw {fname molid args} {
    
    # delete old box
    global my_vmd_boxgraph
    if {[info exists my_vmd_boxgraph]} then {
        foreach g  $my_vmd_boxgraph {
            graphics $molid delete $g
        }
    }
    
    # draw new box 
    graphics $molid color iceblue
    set my_vmd_boxgraph [draw unitcell style rod width 2.5 ]
}

trace variable vmd_frame(0) w do_boxdraw
animate goto 0