RE: Autocorrelation function of Rg goes down without fluctuation around zero

From: Lennart Nilsson (
Date: Thu Aug 18 2016 - 13:29:40 CDT

Are you sure that you have calculated the autocorrelation < (Rg(t) - <Rg>)(Rg(t+T)-<Rg>)>, ie of the Rg *fluctuations*, and not of Rg itself? There is no reason for <Rg(t)Rg(t+T)> to decay to zero; quite the opposite.
I did not find C(t) in the files you posted, only Rg(t).

From: [] On Behalf Of faride badalkhani
Sent: den 18 augusti 2016 17:17
To: Brian Radak <>
Cc: namd-l <>
Subject: Re: namd-l: Autocorrelation function of Rg goes down without fluctuation around zero

Thank you for your answer and time. At first I performed minimization for 20 ps and then NPT simulation for 10 ns. After that, I restarted the simulation for 5 more ns and continued simulation in 5 ns steps till 40 ns. I did not get any error during the simulations, but the autocorrelation function does not fluctuate around zero. I have shared the Rg, RMSD, and C(t) ans also .top, .par, .conf, .pdb, and the .log file for the first 10 ns in the link below:
p.s. Bond distances, angles, dihedrals and impropers were taken form CHARMM GENERAL FORCE FIELD and the charges were assigned using CGen FF on paramchem.
Thank you so much for your help!

On Thu, Aug 18, 2016 at 7:17 PM, Brian Radak <<>> wrote:
You either made a mistake in your calculation or else your statistics are bad. Some details might help distinguish which of those is the case.


On 08/18/2016 04:21 AM, faride badalkhani wrote:
Dear NAMD users,

I have plotted the autocorrelation function of radius of gyration as a function of time to investigate the equilibration of a system of hyperbranched polymer. However, the plot goes down and does not fluctuate around zero.

Could anybody tell me what is the reason and what should I do?
Any help will be appreciated.


Brian Radak
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory

9700 South Cass Avenue, Bldg. 240
Argonne, IL 60439-4854
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