Steered molecular dynamics - umbrella samling

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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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Setting the pull-coord1-groups = 0 is setting the origin as the absolute reference (check this link 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