[gmx-users] Time reversibility and settle

[Berk Hess]

>From: "Francesco Mercuri" <mercuri at email.com>
>Reply-To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>To: gmx-users at gromacs.org
>Subject: [gmx-users] Time reversibility and settle
>Date: Wed, 11 May 2005 07:31:20 -0500
>
>Hi all,
>I'm trying to perform some time-reversibility tests with Gromacs on
>small water clusters. I wonder whether using "settle" for constraining
>the geometry of water molecules still keeps time-reversibility.
>Test calculations give very poor time-reversibility properties.
>In principle, the dynamics of constrained systems could still be symplectic 
>and
>time-reversible (as shown by a few papers of Leimkuhler et al.), but,
>probably, some modifications of the integration scheme are needed in order
>to get a "true" time-reversible MD.
>Does anybody got any experience about that? Or any suggestion to improve
>time-reversibility?

Leap-frog with settle is analytically time reversible.
But numerically it will almost never be time reversible.
Since MD is chaotic trajectories will diverge exponentially,
also the time reverse.
You can make the exponential time constant a lot smaller
by changing from your compilation from single to double precision.

Berk.

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