proc vmd_draw_arrow {mol start end} {
     set middle [vecadd $start [vecscale 0.9 [vecsub $end $start]]]
     graphics $mol cylinder $start $middle radius 0.05
     graphics $mol cone $middle $end radius 0.01 color 3
}

proc vmd_draw_vector {args} {
    set usage {"draw vector {x1 y1 z1} {x2 y2 z2} [scale <s>] [resolution <res>] [radius <r>] [filled <yes/no>]"}
    # defaults
    set scale 2.0
    set res 50
    set radius 0.1
    set filled yes

    if {[llength $args] < 3} {
        error "wrong # args: should be $usage"
    }
    set mol    [lindex $args 0]
    set center [lindex $args 1]
    set vector [lindex $args 2]
    if {[llength $center] != 3 || [llength $vector] != 3} {
        error "wrong type of args: should be $usage"
    }

    foreach {flag value} [lrange $args 3 end] {
        switch -glob $flag {
            scale  {set scale  $value}
            res*   {set res    $value}
            rad*   {set radius $value}
            fill*  {set filled $value}
            default {error "unknown option '$flag': should be $usage" }
        }
    }
    
    set vechalf [vecscale [expr $scale * 0.5] $vector]
    return [list \
        [graphics $mol color yellow]\
        [graphics $mol cylinder [vecsub $center $vechalf]\
            [vecadd $center [vecscale 0.7 $vechalf]] \
              radius $radius resolution $res filled $filled] \
        [graphics $mol color orange]\
        [graphics $mol cone [vecadd $center [vecscale 0.6 $vechalf]] \
   	    [vecadd $center $vechalf] radius [expr $radius * 2.5] \
              resolution $res]]
}

proc vmd_draw_spin {args} {
     global molid
     graphics $molid delete all
     set frame [molinfo $molid get frame]
     set natoms [molinfo $molid get numatoms]
     for {set i 0} {$i < $natoms} {incr i} {
        set sel [atomselect top "index $i"]
#         set sel [atomselect top "index 1200"]
        set coords [lindex [$sel get {x y z}] $molid]
        set velocities [lindex [$sel get {vx vy vz}] $molid]
        draw vector $coords $velocities  
        set uvx [lindex [$sel get {vx}] $molid]
       set uvy [lindex [$sel get {vy}] $molid]
       set uvz [lindex [$sel get {vz}] $molid]
       $sel set user [vecadd [vecadd [vecscale $uvy  $uvy] [vecscale $uvz  $uvz] ] [vecscale $uvx  $uvx]]
       $sel set user $uvy
       #draw vector $coords {0.0 uvy 0.0}
       }
 #pbc box -color 3 
}

proc enable_trace {} {
     global vmd_frame
     trace variable vmd_frame([molinfo top]) w vmd_draw_spin    
     }
     
set molid [mol addfile {/home/jtranch/Documents/lammps/src/dump.lammpstrj} type {lammpstrj} autobonds off first 0 last -1 step 1 waitfor all]
scale by 0.5
animate style Loop
enable_trace