# Steered molecular dynamics - umbrella samling
**URL:** https://gromacs.bioexcel.eu/t/steered-molecular-dynamics-umbrella-samling/13006
**Category:** User discussions
**Tags:** mdp-parameters, mdrun, simulation-setup
**Created:** [December 4, 2025, 11:39am UTC](https://gromacs.bioexcel.eu/t/steered-molecular-dynamics-umbrella-samling/13006 "2025-12-04T11:39:59Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@enthusiast](https://gromacs.bioexcel.eu/u/enthusiast)
#### Post date: [December 4, 2025, 11:39am UTC](https://gromacs.bioexcel.eu/t/steered-molecular-dynamics-umbrella-samling/13006/1 "2025-12-04T11:39:59Z")
Hello everyone,
I do not understand how exactly the umbrella sampling works.
I want to simulate a spring, which is attached to an atom of my protein. I want the other end of the spring to be pulled, which would lead to the attached atom to be dragged along.
But I always need to define two groups for the spring, right? Or can I not define pull-coord1-groups and then just apply the harmonic restraint to one atom? (But how would that work, if a spring always needs two ends?)
I saw in some code that the defined pull-coord1-groups = 0 1, but did not define 0 anywhere- however, I do not exactly understand what 0 would mean. Is there a possibility to choose a reference that is being connected through a spring to an atom without actually adding an atom to the system?
Thank you all for your responses and help in advance!
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### Author:  [@Karis](https://gromacs.bioexcel.eu/u/Karis)
#### Post date: [December 8, 2025, 7:36am UTC](https://gromacs.bioexcel.eu/t/steered-molecular-dynamics-umbrella-samling/13006/2 "2025-12-08T07:36:00Z")
Setting the pull-coord1-groups = 0 is setting the origin as the absolute reference (check this [link](https://manual.gromacs.org/documentation/2018/user-guide/mdp-options.html#com-pulling) for a description of the mdp parameters) the groups you would need to define would depend on what you’re setting for pull-coord1-geometry, if you’re setting that to distance, then you would need to define 2 groups