mol load psf Bmim_Tf2N.psf dcd abf_nvt_Bmim_Tf2N.dcd

set nframes [molinfo top get numframes]
set outfile [open frames.dat w]

set xi 3.33
set er 0.1

for { set j 0 } {$j < $nframes} {incr j} {

    set atom1 [atomselect top "resid 1 and name C2" frame $j]
    set xyz1 [lindex [$atom1 get {x y z}] 0]
    
    set atom2 [atomselect top "resid 2 and name N1" frame $j]
    set xyz2 [ lindex [$atom2 get {x y z}] 0]
    
    set hd [vecsub $xyz1 $xyz2]
    set vl [veclength $hd]

    if { $vl < $xi + $er } {

        if { $vl > $xi - $er } {

            puts $outfile $j

        }

    }

}

mol delete all
close $outfile
exit