[gmx-users] Are the output of velocity, force and position at the same time

[qiao rui]

hi,
	Maybe this is a old question, but I am curious whether the
velocity and position output by Gromacs are at the same time. when I
looked at the md.c, it reads:

	...
	do_force();
	...
	write_trj();
	...
	update();
	...

	Since Gromacs is using a leap-frog algorithm, the velocity and
position are not available at the same time, I tend to guess that unless
there are some interpolation in write_trj(), the output of velocity, force
and position would not be at the same time. So I would guess that:
	velocity (t-dt/2)
	position (t)
	force    (t+dt)

	In addition, has the output force considers the impact of the
constraints?

	Does this make sense or  I am mising something here?

	Thanks a lot.

	sincerely,

	Rui Qiao
------------------------
Rui Qiao
Research Assistant
Beckman Institute, UIUC