PULL CODE: applying force

GROMACS version: 2020.1
GROMACS modification: Yes/No
In order to apply the periodically changed force, which option should I use in the pull-coord1-type?

I don’t understand exactly what you mean with “periodically changed force”. You can continuously change the reference distance with pull-coord1-rate.

Thank you for your reply. I mean the external force like vibration. Is it possible to pull molecule in back and forth?

No, this is not possible. It is not difficult to change this in the code though.