[gmx-users] Using umbrella sampling at constant force

[Sanku M]

Hi,
  I am trying to compute free energy of a polymer in presence of a constant force.
If I understand correctly, I need to use two pull-option here in gromacs: one for performing umbrella sampling and other for applying  constant force. But, as far as I know I can not specify two pull option in single mdp file of gromacs. 
So, I was wondering whether there is any way out in gromacs regarding this issue.
Thanks
Sanku