We have a complex with two chains. From one chain (chain A), we are
fixing the COM. We are pulling on the COM of the other chain (chain
B). Now assume that due to interactions with chain B, chain A is
elongated in the direction of pulling. This means that the COM of
chain A has moved in the directions of chain B. To keep the COM of
chain A at the same position, the 'rear end' of chain A has to go in
the opposite direction than the 'front end', so that chain A
experiences additional forces and is even more (and now not due to
'real' interactions, but artificially ) elongated due to the COM
fixing. Did I understand that correctly?
Thanks,
Kay.
On Jul 18, 2006, at 11:31 AM, Berk Hess wrote:
From: Kay Gottschalk <[EMAIL PROTECTED]>
Reply-To: Discussion list for GROMACS users <gmx-users@gromacs.org>
To: Discussion list for GROMACS users <gmx-users@gromacs.org>
Subject: [gmx-users] center of mass pulling in AFM
Date: Mon, 17 Jul 2006 12:58:02 +0200
Hi there,
we are doing pulling simulations of protein complexes. For these
simulations, we keep the COM of one group fixed and pull on the
COM of the other group. Now we got into this discussion, whether
this introduces artificial distortions on the hold protein. If
the protein deforms into the direction of the force vector, the
center of mass changes. Does this mean that a force on the other
end of the protein is applied, so that the other end also deforms
in order to keep the center of mass fixed? Or how is the COM held
fixed ?
This all depends on what you are keeping fixed.
Of which groups are you keeping the COM fixed?
Since MD preserves momenta, you can only
fix distances between COMs by adding forces,
which is exactly what the pull code does.
Additionally fixing more than one COM will
introduce additional forces.
Berk.
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