How to set bond length distance

[khariyahA]

Hi everyone,
I am trying to built carbon doped boron nitride nanoribbon, i am trying to set the bond length to a specific distance. For example I want to set bond length carbon to carbon to 1.42 A , B to N to 1.45 A, carbon to N to 1.32 A and B to carbon to 1.55 A. I appreciate if anyone can guide me on this.
Thank you 

[Anders Blom]

I am not sure why you want to set the C-C distance in a B-N structure, but you can easily create B-N nanoribbons with the desired bond length 1.45 using the NanoRibbon tool in the builder

As for C-B and C-N, it will be impossible to impose those distances just locally, because also the B-N distances will change around the dopant.
Rather, they should be the result of running a geometry optimization on the structure where you have replaced certain atoms by Carbon (I assume you are talking about substitutional doping, based on the distances mentioned).
Fortunately, the forcefield ReaxFF_CHBN_2015 seems to do a decent job so you can use the Quick Optimizer to at least get a good starting structure. But depending on the width of the nanoribbon and the position of the C atom, all bond lengths at least 2-3 neighbors out will be changed (and in fact, even the edge atoms will be moving a bit).