/usr/share/xcrysden/util/unitconv is in xcrysden-data 1.5.60-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 | #!/bin/sh
# \
# this line restarts tclsh \
exec tclsh $0 "$@"
#
# Usage: unitconv unit value 
#    or: unitconv unit expr
#
# 
# supported units
set energy_units {
    au    
    ry    
    ev
    kjm    
    kcalm
    kj
    kcal
    erg
}
set length_units {
    angs
    bohr
    m
}
set allowed_units "$energy_units $length_units"
# check for correctness of specified unit on command-line
set unit [string tolower [lindex $argv 0]]
if { [lsearch -nocase $allowed_units $unit] < 0 } {
    puts stderr "input unit not known; must be one of [join $allowed_units {, }]"
    exit
}
# ------------------------------------------------------------------------
# PHYSICAL CONSTANTS
#
# Physical constants, SI (NIST CODATA 2006), Web Version 5.1
# http://physics.nist.gov/constants
set j   1.000000000000
set au  4.35974394e-18;  # J
set ev  1.602176487e-19; # J
set mol 6.02214129e23
# thermo-kilocalorie (International Standard ISO 31-4: Quantities and
# units – Part 4: Heat. Annex B (informative): Other units given for
# information, especially regarding the conversion
# factor. International Organization for Standardization, 1992.)
set kcal 4184;  # J
# from wikipedia:
set erg 1e-7; # J
#--------------------------------
# Energy Units (use J as default)
#--------------------------------
set kj     1000.00000000000000
set ry     [expr $au   / 2.0]
set kjm    [expr $kj   / $mol]
set kcalm  [expr $kcal / $mol]
#-----------------------------------------
# Distance Units (use Angstrom as default)
#-----------------------------------------
set angs 1.0
set bohr 0.52917720859
set m    1.e10
# ------------------------------------------------------------------------
#
# evaluate & convert user specified expression/value
#
upvar #0 $unit un_fac
# parse the command line arguments
foreach item [lrange $argv 1 end] {
    append numbers [format "%s " $item]
}
set orig_value [expr 1.0 * ($numbers)]; 
# store result either in J or Angstroms !!!
set value [expr $un_fac * $orig_value]; 
#
# PRINTOUT
#
puts stdout "$orig_value $unit = "
if { [lsearch -nocase $energy_units $unit] > -1 } {
    # do we have energy ?
    
    foreach _un $energy_units {
	upvar #0 $_un unval
	set val [expr $value / $unval]
	puts stdout "    = $val $_un"
    }
} elseif { [lsearch -nocase $length_units $unit] > -1 }  {
    # ... or length ?
    foreach _un $length_units {
	upvar #0 $_un unval
	set val [expr $value / $unval]
	puts stdout "    = $val $_un"
    }
}
exit 0
 |