sub total_energy {
    my $totalE = 0;
    foreach my $i ( 0 .. $#arrayx ) {
        foreach my $j ( $i + 1 .. $#arrayx ) {

            my $r = sqrt(
                ($arrayx[$i] - $arrayx[$j])**2 +
                ($arrayy[$i] - $arrayy[$j])**2 +
                ($arrayz[$i] - $arrayz[$j])**2
            );
            $totalE = $totalE + vdw( $r );
            return $totalE;
        }
    }
}