Have you watched the trajectory? That will show you what your pull group is doing. Is this a periodicity effect? It should be fairly obvious iff the sign of dX, etc flip-flops in the pullx.xvg file.

-Justin

Gavin Melaugh wrote:
Thanks Chris and Justin

I do get the output I want from grompp(below). The distance of 5.507 nm
is the distance that I want to start with and I assume it is therefore
the equilibrium distance for the harmonic potential. The COM distances
from the simulations fluctuate around this value which is what I want,
although my histogram is the wrong shape.

Pull group  natoms  pbc atom  distance at start     reference at t=0
       0               84        42
1 84 126 5.507 5.507

Cheers

Gavin

chris.ne...@utoronto.ca wrote:
I missed that pull_start = yes, sorry. So Gavin, your run should have
stayed around the starting position. Make the histogram directly and
you should get a better idea.

Chris.

-- original message --


To clarify, the tutorial sets "pull_init1 = 0" because it is used in
conjunction
with "pull_start = yes," in order to take the starting COM distance as
the
reference.  Check the grompp output to verify that you're getting what
you think
you should be.  Otherwise, you'd have to set "pull_start = no" and
modify your
.mdp file to reflect the proper value of pull_init1 for every window.

-Justin






--
========================================

Justin A. Lemkul
Ph.D. Candidate
ICTAS Doctoral Scholar
MILES-IGERT Trainee
Department of Biochemistry
Virginia Tech
Blacksburg, VA
jalemkul[at]vt.edu | (540) 231-9080
http://www.bevanlab.biochem.vt.edu/Pages/Personal/justin

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