[gmx-users] Constraint the COM distance during Umbrella Sampling

[Giuseppe Léonardo Licari]

Dear all,
I did a stirring simulation followed by an umbrella sampling run in order to get a free energy of interaction between two solutes. The two solutes have a negative and positive charge respectively. Now, I would like to rerun the simulations with a different set of FF parameters for the two solutes. In the new set of parameters the two solutes are neutral. However, during the umbrella sampling simulation the COM distance between the two solutes is going too far from the set initial value. I tried to increase a lot the force constant of the harmonic potential. I also tried to change "umbrella" to "contraint", with no success. How can I fix the COM distance between the two solutes? It seems that increasing the force constant does not work. Am I making some mistake in the input? I have Gromacs v. 5.1.2.

Here there is my input mdp file: https://www.dropbox.com/s/pg55zpoouvldm9e/pull_md_10ns.mdp?dl=0

Thanks for the help and have a nice day.
Regards,
Giuseppe Licari