# Pull-force error **URL:** \ **Category:** User discussions\ **Tags:** mdp-parameters, umbrella-sampling, pull-code\ **Created:** [November 14, 2024, 1:30pm UTC](https://gromacs.bioexcel.eu/t/pull-force-error/10556 "2024-11-14T13:30:05Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![pratyush](https://avatars.discourse-cdn.com/v4/letter/p/cc9497/32.png) [@pratyush](https://gromacs.bioexcel.eu/u/pratyush)\ **Post date:** [November 14, 2024, 1:30pm UTC](https://gromacs.bioexcel.eu/t/pull-force-error/10556/1 "2024-11-14T13:30:05Z")
GROMACS version: 2022 GROMACS modification: Yes/No So I am doing pull-force calculation of a protein-protein complex where I have to pull the smaller protein from the larger subunit. The pullf.xvg curve seems fine to a certain extent and goes down towards zero very usually but around 40 kJ/mol it just fluctuates around and does not converge to zero. What could this mean? I am attaching the plot as well as my pull code. Kindly help. ![Screenshot 2024-11-14 152218](https://europe1.discourse-cdn.com/flex017/uploads/bioexcel1/original/2X/8/877af01f78f757c66bbc1e417bf72ee926ce7d20.png) ; Pull code pull = yes pull\_ncoords = 1 ; only one reaction coordinate pull\_ngroups = 2 ; two groups defining one reaction coordinate pull\_group1\_name = chain\_B pull\_group2\_name = chain\_A pull\_coord1\_type = umbrella ; harmonic potential pull\_coord1\_geometry = distance ; simple distance increase pull\_coord1\_dim = Y N N pull\_coord1\_groups = 1 2 pull\_coord1\_start = yes ; define initial COM distance \> 0 pull\_coord1\_rate = 0.01 ; 0.01 nm per ps = 10 nm per ns pull\_coord1\_k = 1000 ; kJ mol^-1 nm^-2 pull-pbc-ref-prev-step-com = yes ---
**Author:** ![MagnusL](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/magnusl/32/2380_2.png) [@MagnusL](https://gromacs.bioexcel.eu/u/MagnusL)\ **Post date:** [November 15, 2024, 6:25am UTC](https://gromacs.bioexcel.eu/t/pull-force-error/10556/2 "2024-11-15T06:25:32Z")
That does not look like an error. The pull force means that the groups that you are pulling are lagging behind the pull reference position. If you add `pull-print-com = yes` and `pull-print-ref-value = yes` to your mdp options you can see this in the pullx.xvg file. The reason behind this is that there is a friction when pulling a protein in water, even if the protein is small. ---
**Author:** ![pratyush](https://avatars.discourse-cdn.com/v4/letter/p/cc9497/32.png) [@pratyush](https://gromacs.bioexcel.eu/u/pratyush)\ **Post date:** [November 15, 2024, 7:49am UTC](https://gromacs.bioexcel.eu/t/pull-force-error/10556/3 "2024-11-15T07:49:57Z")
Thank you for your reply. What do you think will solve it? I mean at any point of time, the force has to become zero, right? ---
**Author:** ![MagnusL](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/magnusl/32/2380_2.png) [@MagnusL](https://gromacs.bioexcel.eu/u/MagnusL)\ **Post date:** [November 15, 2024, 7:51am UTC](https://gromacs.bioexcel.eu/t/pull-force-error/10556/4 "2024-11-15T07:51:36Z")
I don’t see why it will have to become 0, except if you are pulling in vacuum. ---
**Author:** ![pratyush](https://avatars.discourse-cdn.com/v4/letter/p/cc9497/32.png) [@pratyush](https://gromacs.bioexcel.eu/u/pratyush)\ **Post date:** [November 15, 2024, 8:00am UTC](https://gromacs.bioexcel.eu/t/pull-force-error/10556/5 "2024-11-15T08:00:19Z")
I had almost come to the same conclusion to be very honest but I was advised by someone to look for a possible solutions bcs according to them it always converged to zero in their case. Is there some paper or documentation I can read to have a good understanding on this? Thank you for the help. ---
**Author:** ![MagnusL](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/magnusl/32/2380_2.png) [@MagnusL](https://gromacs.bioexcel.eu/u/MagnusL)\ **Post date:** [November 15, 2024, 8:09am UTC](https://gromacs.bioexcel.eu/t/pull-force-error/10556/6 "2024-11-15T08:09:12Z")
Was their molecule as large as yours? Did they pull as quickly? Have a look at [Nonequilibrium Measurements of Free Energy Differences for Microscopically Reversible Markovian Systems | Journal of Statistical Physics](https://link.springer.com/article/10.1023/A:1023208217925). What you are observing is the dissipated force.