# Rate and force unit conversion for pulling paramters

**URL:** <https://gromacs.bioexcel.eu/t/rate-and-force-unit-conversion-for-pulling-paramters/10271>\
**Category:** User discussions\
**Tags:** pull-code\
**Created:** [October 1, 2024, 3:42pm UTC](https://gromacs.bioexcel.eu/t/rate-and-force-unit-conversion-for-pulling-paramters/10271 "2024-10-01T15:42:12Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![com\_gro](https://avatars.discourse-cdn.com/v4/letter/c/f475e1/32.png) [@com\_gro](https://gromacs.bioexcel.eu/u/com_gro)\
**Post date:** [October 1, 2024, 3:42pm UTC](https://gromacs.bioexcel.eu/t/rate-and-force-unit-conversion-for-pulling-paramters/10271/1 "2024-10-01T15:42:12Z")

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Hello good people,

I need your help! I know this is very basic, but I want to cross-check if my unit conversions are correct, as I need to use these values for the following two parameters in my simulation.  
I want to pull C-terminal at constant velocity of 10^5 μm/s by using a harmonic spring with the force constant of 5 × 10^4 pN/μm. Can I use it like this, if so is my conversion correct?

The default units for the parameters are:  
pull-coord1-rate = [nm/ps]  
pull-coord1-k = [kJ mol^-1 nm^-2]

I’m trying to convert to different units and came up with the following:

1. 10^5 μm/s = 10^-4 nm/ps
2. 1 pN/μm = 6.022 × 10^-11 kJ/mol/nm²  
so, 5 × 10^4 pN/μm = 3.011 × 10^-6 kJ/mol/nm²  
Codes:

; Pull code  
pull = yes  
pull\_ngroups = 2 ; Two groups involved  
pull\_group1\_name = group1  
pull\_group2\_name = group2  
pull-group1-pbcatom = 3750  
pull-pbc-ref-prev-step-com = yes  
pull\_ncoords = 1 ; One coordinate for pulling

; Define pull coordinates  
pull-coord1-type = umbrella ; Umbrella pulling  
pull-coord1-groups = 1 2 ; 1 for reference, 2 for the atom to be pulled  
pull-coord1-geometry = distance ; Distance geometry  
pull-coord1-dim = N Y N ; Only consider the Y component of the distance  
pull-coord1-start = yes ; Start from the initial configuration  
pull-coord1-rate = ??? ; nm per ps  
pull-coord1-k = ??? ; Force constant in kJ mol^-1 nm^-2 , **need to use (5 × 10^4 pN/μm)**

Thanks in advance!

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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:** [October 2, 2024, 6:13am UTC](https://gromacs.bioexcel.eu/t/rate-and-force-unit-conversion-for-pulling-paramters/10271/2 "2024-10-02T06:13:21Z")

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Hi,  
Before doing any unit conversions, please have a look at [Molecular dynamics parameters (.mdp options) - GROMACS 2024.3 documentation](https://manual.gromacs.org/documentation/current/user-guide/mdp-options.html#mdp-pull-coord1-k), specifically “For constant force pulling this is the force constant of the linear potential, and thus the negative (!) of the constant force in kJ mol-1 nm-1 (or kJ mol-1 rad-1 for angles).”

I think that will save you some trouble.