Thank you guys for your answers, Bruce, you said: "The central point here is that Ifeffit (and by extension, Artemis) allows you to write such things as the distance between atoms in the form of an almost arbitrary algebraic expression." So I understand that I can just tell Ifeffit/Artemis: distance between Atom#1 and Atom#2 is equal to "x" Angstroms, and that will be forced for all the next calculations. Did I understand well?? If this is true I believe that this way is easier than build a lot of structures to test (If you have more than one histidine the number of structures increases...). Anyway, I had a look at many .inp the examples but I did not understand well how to give this command to the program. Can you write me an example? It would be much easier for me to start from that example and add many atom-atom distance restraints to keep a rigid structure. Thank you very much Max