[gmx-users] about the velocity output from leap-frog integrators

[Ravi Bhadauria]

Hello users,

I have few conflicting answers from the user mailing list about the
following question.

Using leap-frog integrator, what is the velocity that is WRITTEN in
the trajectory file for a frame corresponding to r(t)? Is it v(t-dt/2)
or v(t). Has the protocol for writing velocity in trajectory file been
consistent from version 3.x to 4.5.x?

Here are few posts hinting its v(t):
http://lists.gromacs.org/pipermail/gmx-users/2002-July/001969.html
http://lists.gromacs.org/pipermail/gmx-users/2003-September/006956.html

Here are couple of posts hinting its v(t-dt/2):
http://lists.gromacs.org/pipermail/gmx-users/2006-June/022261.html
http://lists.gromacs.org/pipermail/gmx-users/2003-March/004827.html

Please let me know. The problem is with certain class of simulations
of bounded flows, these two definitions provide contrasting results
for suitably averaged velocity.

Sincerely
Ravi Bhadauria