blen 
"all"|<name>|<number>
,"all"|<name>|<number>,<b1>,<b2>
The simple form of this command calculates the bond length and its
standard deviation between the atoms number <a1> and <a2>. Note
that the 'real' distance within the crystal is given and *no*
periodic boundary conditions are applied.
The second command calculates the bond lengths between the atoms selected
by the first two parameters that fall in the range specified by
<b1> and <b2>. As usual atoms can be selected by name or type number
or simply use "all" to select all atoms. Here periodic boundaries
are applied like for the PDF calculation itself. Thus the results
of the two versions of 'blen' might be different for the same atom
number pair !