[gmx-users] Using umbrella sampling at constant force

[Christopher Neale]

You can not have multiple reference groups or multiple types of pulling (constraint, harmonic, etc) in any version of gromacs that I am aware of.  I have modified versions of gromacs 3.3.3 and 4.0.5 that we modified in-house to put a loop over the pull code so that this is possible. For v3.3.3, it is very simple. For v4.x it is much harder to modify this code and also we did not end up coding the ability to use different types of pulling, just the ability to have different reference groups, etc. I can send you the code for v3.3.3 if you would like. The code for v4.0.5 is part of a manuscript that is not yet published, but contact me in a few months and we may be able to send you that code as well.

Chris.

-- original message --

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