[DFTB-Plus-User] Constraint for bonds

[Wagner Müller]

Hi DFTB+ community!
I’ve recently started working with DFTB+ and am currently trying to simulate a phosphoethanolamine group. In this group, nitrogen carries an additional hydrogen (giving it a positive charge), and there is an oxygen atom with only a single bond (making it negatively charged). However, when I run the geometry optimization, the extra hydrogen on the nitrogen atom always moves to the oxygen atom.
I was wondering if it's possible to apply constraints specifically to the bond distance between the nitrogen and hydrogen atoms to prevent this behavior. I am aware that I can constrain individual atoms (such as N or H), but I would like to know if there is a way to specifically constrain the N-H bond distance.
Thanks in advance for your help!
Best regards, Wagner

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