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

[Angela Marcela Murcia Rios]

Thank you for the clarification!


-Angela


________________________________
From: gromacs.org_gmx-users-bounces at maillist.sys.kth.se <gromacs.org_gmx-users-bounces at maillist.sys.kth.se> on behalf of Alex <nedomacho at gmail.com>
Sent: June 22, 2017 7:57 PM
To: Discussion list for GROMACS users
Subject: Re: [gmx-users] Constraint vs. Umbrella in Pull COM

In the case of SHAKE constraint, there is no spring-like physics, like in
umbrella pulling. The distance between COMs evolves according to the rate
you set, but there is no COM oscillation around prescribed values, SHAKE
turns the distance into a solid stick.

Alex

On Thu, Jun 22, 2017 at 5:13 PM, Angela Marcela Murcia Rios <amurciar at uwo.ca
> wrote:

> 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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