[gmx-users] pull=constraint gives zero forces

[alex.bjorling]

Hi,

I'm using the pull code to maintain the initial structure of a protein
that otherwise deforms. Using pull=umbrella does what I expect it to,
but pull=constraint produces zero forces. I'm using version 4.5.5 with
the MARTINI force field.

The pull=umbrella mdp contains the following,


and gives the following pullf output:


For such runs, the group COM:s stay within 0.1 nm of their initial
positions, throughout a long trajectory.

The pull=constraint mdp starts with



and produces the pullf output



For these runs, the group COM:s move around freely. I guess I'm doing
something wrong but can't work out what. I've tried specifying k:s for
the contraint runs, and tried removing all other constraints in the
molecule. I've tried to comply with the manual's instructions but to
no avail.

Any ideas?

Cheers,
Alexander Björling,
PhD candidate, University of Gothenburg, Sweden




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