[gmx-users] Regarding constant force pulling

[Singam Karthick]

Dear all
I am trying to calculate rate of unfolding from pulling simulation. I have seen a paper in PLOS one (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0034781) which says they have done a constant force pulling from 750 to 3000 pN. I have a small query how do we specify the constant force in mdp file? I can see a pull_rate but how to specify the rate in pN. 
regards
singam