Calculating Rg using VMD

I am working on a system with 60 copies of a molecule which has 1287 atoms in it. The script below loads the trajectory and calculates the radius of gyration of each copy.

mol new system.gro

for {set num 0} {$num < 201} {incr num} {
mol addfile cen_md_$num.xtc waitfor all
}

for {set i 1} {$i <61} {incr i} {
set start [expr 1 + 1287 * ($i-1) ]
set end [expr 1287 * $i]
puts "$start $end"
set sel [atomselect top "serial $start to $end"]
set filename [open "rg-dextran-number-$i.txt" w]
puts "Number of frames is [molinfo top get numframes]";
for {set frame 0} {$frame < [molinfo top get numframes]} {incr frame} {
animate goto $frame
$sel update
puts $filename [format "%8.3f" [measure rgyr $sel]]
}
close $filename
}

quit