Thanks Berk, 4.0.4 does show the expected behaviour for me with the
twin-range cutoff. Since I obtained 4.0.4 prior to mentioning this
issue, I gather that you gave the free energy code a good looking-over
recently. I really appreciate it.
I have seen only one previous paper that utilized a twin-range cutoff
for annihilation simulations, but that was done via gromos in 2003
(Lei and Smith, Biophys. J., 3513-3520, 2003) and since this part of
the free-energy code in gromacs is done in C, not fortran, I expect
that gromos annihilation simulations utilizing a twin-range cutoff are
not necessarily subject to any of these issues.
Thanks again,
Chris.
-- original message --
Hi,
I just fixed this in 4.0.4.
The forces are always correct, dgdl in 4.0.3 was only correct at
nstlist steps.
Currently 4.0.4 is only available through ftp.
We still need to add it (and the release notes) to the download page.
Berk
Date: Tue, 17 Feb 2009 17:50:17 -0500
From: chris.neale at utoronto.ca
To: gmx-users at gromacs.org
Subject: [gmx-users] anomalous free energy dgdl.xvg values every
nstlist steps while using a twin-range cutoff Hello,
I believe that the free-energy code dgdl contribution from the
twin-range cutoff is being calculated incorrectly in gromacs 4.0.3
(and probably other versions as well).
Specifically, I notice that the dgdl values spike at nstlist
intervals. This can be seen directly from my dgdl.xvg and also from
running g_analyze -ac.
I suspect that while the forces from the LJ interactions that are in
the longer range of the twin-range are added every step while the
dgdl value is modified to have nstlist times these contributions
every nstlist step.
My belief that the forces are truley added every step comes from
section 4.6.3 of the gromacs 4 manual:
"In the neighbor list all interaction pairs that fall within rlist
are stored. Furthermore, the interactions between
pairs that do not fall within rlist but do fall within
max(rcoulomb,rvdw) are computed during NS, and the forces
and energy are stored separately, and added to short-range forces at
every time step between successive NS."
Perhaps the long-range LJ component forces are actually applied to
the particles every nstlist steps as a force multipled by nstlist,
as is done in NAMD, but I am currently under the impression that
this is not the case.
I am happy to provide more details and files if necessary, but
hopefully this information is sufficient for a skilled coder to take
a look and determine if this is indeed the case.
Thank you,
Chris.
-- snip --
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