[gmx-users] umbrella sampling

[spss4 at iacs.res.in]

  Hi all
I am trying to do umbrella sampling of protein unfolding with radius of
gyration as reaction coordinate using gromacs-2016.3 patched with
plumed-2.3.2. I want to calculate free energy for the system. I am using
the following command

gmx wham P=1.4 1.5 100 0.001 300 0 metadata.dat  free.dat

It shows an error 
  Fatal error:
  Give either pullx (-ix) OR pullf (-if) data. Not both.

But I am using radius of gyration as variable.Why I need to give pullx ?
Please anyone help.
   
Thanks