On 10/13/12 3:17 PM, Sanku M wrote:
Hi all,
   I am trying to do an umbrella sampling of a fairly extended state of the 
protein along end-to-end distance of it. I am using a rectangular box ( 
elongated along x ) of dimension 20 nm X 8nm X 8 nm. I am doing the simulation 
in NVT ensemble. gromacs 4.5.4 is being used.

But, even after running for 1 ns, the simulation crashes saying that

Program mdrun_5mpi, VERSION 4.5.4
Source code file: pull.c, line: 329

Fatal error:
Distance of pull group 1 (2.940209 nm) is larger than 0.49 times the box size 
(9.000000)


But if I see the trajectory, it hardly shows any issue with seeing the PBC 
image . I am not sure how to avoid this issue.

The pull code is very sensitive to these motions. Mathematically, I don't understand why mdrun is complaining, but your setup is prone to failure. An extended protein in an elongated box will surely interact with its periodic image at some point.

Should I make one of the endgroup as freeze group so that it does not get 
tilted.

I don't think you'll be able to freeze or restrain enough of the protein to make a difference that does not significantly impact the dynamics of the system.

-Justin

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

Justin A. Lemkul, Ph.D.
Research Scientist
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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