I've been using ifzmat and fvalue functions in the $zmat input card to constrain the distance between two atoms during geometry optimization, for the purpose of predicting a transition state. When I set the fvalue distance, say to 2.0 (angstroms), does this absolutely fix the interatomic distance, or indicate a maximum permitted distance? Hypothetically, if the two atoms "wanted" to be closer than 2.0 angstroms, could they be? Does such a function even exist?
Thank you for your time and thoughts!