【答案】应助回帖

set firstframe 500
set lastframe [molinfo top get numframes]

set ooo [atomselect top "name OW"]
set totalo 0

for {set h 40 } { $h <= 200 } { incr h } { set count($h) 0 }
# 0.4 to 2.0 nm step by 0.01

for { set f $firstframe} { $f < $lastframe } { incr f } {
    $ooo frame $f
    foreach z [$ooo get z] {
            incr count([expr round($z*10)]) ;# z in Ang
                if {$z < 20.0} {incr totalo}
}   }

set outf [open "profile.dat" w]
for {set h 40 } { $h <= 200 } { incr h } {
    puts $outf [format "%.2f\t%8.6f" [expr $h/100.0] \
        [expr 1.0*$count($h)/$totalo]]
}
close $outf