# Pulling force keeps increasing

**URL:** <https://gromacs.bioexcel.eu/t/pulling-force-keeps-increasing/9398>\
**Category:** User discussions\
**Created:** [June 14, 2024, 11:58pm UTC](https://gromacs.bioexcel.eu/t/pulling-force-keeps-increasing/9398 "2024-06-14T23:58:47Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![tranle](https://avatars.discourse-cdn.com/v4/letter/t/ebca7d/32.png) [@tranle](https://gromacs.bioexcel.eu/u/tranle)\
**Post date:** [June 14, 2024, 11:58pm UTC](https://gromacs.bioexcel.eu/t/pulling-force-keeps-increasing/9398/1 "2024-06-14T23:58:48Z")

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GROMACS version: 2023.3  
GROMACS modification: No  
Here post your question

I am trying to calculate the friction force in a system where a molecule is subjected to a harmonic spring in the X and Y dimensions and is pulled along the X-axis on a rough polymer slab (see attached figure). The polymer slab is frozen during this process, and the system was equilibrated before performing the pulling simulation. I expected the force data from `pullf.xvg` to represent the friction force. However, the force appears to increase linearly over time. I am unsure if there is an issue with my input parameters (provided below). Has anyone encountered a similar problem or have any suggestions for resolving this?  
Thank you.

integrator = md  
dt = 0.001   
nsteps = 2000000

nstxout = 1000   
nstvout = 1000   
nstenergy = 1000   
nstlog = 1000   
nstxout-compressed = 10000  
compressed-x-grps = System

cutoff-scheme = Verlet  
nstlist = 10   
rcoulomb = 1.4   
rvdw = 1.4   
pbc = xyz  
DispCorr = no

coulombtype = PME   
pme\_order = 4   
fourierspacing = 0.12

tcoupl =Nose-Hoover  
tc-grps = system  
tau\_t = 0.2  
ref\_t = 298

pcoupl = no

continuation = no   
constraint\_algorithm = lincs   
constraints = none   
lincs\_iter = 1   
lincs\_order = 4   
lincs\_warnangle = 30  
morse = no

gen\_vel = no   
gen-temp = 298  
gen-seed = -1

;\*\*\*\*\*\*\*\*\*\*\*\*\*  
; Pull code \*  
;\*\*\*\*\*\*\*\*\*\*\*\*\*  
pull = yes  
pull-ncoords = 1  
pull-ngroups = 1  
pull-nstxout = 1000  
pull-nstfout = 1000  
pull-xout\_average = no  
pull-fout\_average = no

pull-group1-name = PTM  
pull-coord1\_type = umbrella  
pull-coord1\_geometry = direction  
pull-print-components=yes  
pull-coord1\_dim = Y Y N  
pull-coord1\_groups = 0 1  
pull-coord1-origin = 0.0 0.0 0.0  
pull-coord1\_vec = 1.0 0.0 0.0  
pull-coord1\_start = yes  
pull-coord1\_rate = 0.05  
pull-coord2\_k = 700

freezegrps = PPN  
freezedim = Y Y Y

 ![image](https://europe1.discourse-cdn.com/flex017/uploads/bioexcel1/original/2X/f/f81a204a5fbe29f5c21ff78bfa192ef4724b7ff3.jpeg)  
 ![image](https://europe1.discourse-cdn.com/flex017/uploads/bioexcel1/original/2X/1/1409d6be23b62516f5638e08f14c43319cb6d24d.png)

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<div class="post-metadata">

**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:** [June 15, 2024, 12:24pm UTC](https://gromacs.bioexcel.eu/t/pulling-force-keeps-increasing/9398/2 "2024-06-15T12:24:37Z")

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If the polymer slab is frozen it is quite probable that the group you pull will get stuck on it. The high pull force means that the reference position (the other end of the harmonic spring) is far away from the group you pull. Why do you want the polymer slab to be frozen?

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<div class="post-metadata">

**Author:** ![tranle](https://avatars.discourse-cdn.com/v4/letter/t/ebca7d/32.png) [@tranle](https://gromacs.bioexcel.eu/u/tranle)\
**Post date:** [June 17, 2024, 3:49pm UTC](https://gromacs.bioexcel.eu/t/pulling-force-keeps-increasing/9398/3 "2024-06-17T15:49:22Z")

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The pull group continues to move on the polymer slab. If I don’t freeze the polymer slab, the pull group will drift away from it. I’m unsure if position restraining the polymer is a better option.