[gmx-users] Constraint vs. Umbrella in Pull COM

Angela Marcela Murcia Rios amurciar at uwo.ca
Fri Jun 23 01:13:32 CEST 2017


Hi All,


I was wondering if someone knows the difference between constraint versus umbrella pulling. I have read the manual but I'm still not sure what this means.


1. Umbrella pulling: A harmonic potential is applied between the centers of mass of two groups. Thus, the force is proportional to the displacement.
2. Constraint pulling: The distance between the centers of mass of two groups is constrained. The constraint force can be written to a file. This method uses the SHAKE algorithm but only needs 1 iteration to be exact if only two groups are constrained.

What does it mean by "The distance between the centers of mass of two groups is constrained"? Is this distance fixed? or is the reference group assumed to be fixed?

Thanks for the help!

-Angela



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