VMD script to pick a box within a box

set cellxmin [lindex [lindex [measure minmax $all] 0] 0]
set cellxmax [lindex [lindex [measure minmax $all] 1] 0]
set cellymin [lindex [lindex [measure minmax $all] 0] 1]
set cellymax [lindex [lindex [measure minmax $all] 1] 1]
set cellzmin [lindex [lindex [measure minmax $all] 0] 2]
set cellzmax [lindex [lindex [measure minmax $all] 1] 2]

for {set i 0} {$i < 100} {incr i} {
# This lets you pick a random box of size 10x10x10 in the simulation box
set xmin [expr $cellxmin + ( ($cellxmax-$cellxmin-$boxsize) * rand())]
set ymin [expr $cellymin + ( ($cellymax-$cellymin-$boxsize) * rand())]
set zmin [expr $cellzmin + ( ($cellzmax-$cellzmin-$boxsize) * rand())]
set xmax [expr $xmin + $boxsize]
set ymax [expr $ymin + $boxsize]
set zmax [expr $zmin + $boxsize]

set selection [atomselect top "(x>$xmin and x<$xmax and y>$ymin and y<$ymax and z>$zmin and z<$zmax)"]
# This gives the number of atom in that selection
set n [$selection num];
#This gives the number of non-bonded interactions in the 10x10x10 box selected.
puts $file [expr $n * ($n -1)/2 ]
}