[gmx-users] Umbrella sampling: issue with COM distance far from initial value

[Giuseppe Léonardo Licari]

Dear all,

I have troubles with umbrella sampling. 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 and here you can find the files of a test simulation:

https://www.dropbox.com/s/cl7au3twx8d6iip/test.zip?dl=0

As you see from the trajectory the two solutes go very far from the initial COM distance. Does somebody have an idea about this?

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