**From:** faride badalkhani (*farideh.khamseh_at_gmail.com*)

**Date:** Thu Aug 18 2016 - 11:16:57 CDT

**Next message:**Brian Radak: "Re: Autocorrelation function of Rg goes down without fluctuation around zero"**Previous message:**Brian Radak: "Re: Autocorrelation function of Rg goes down without fluctuation around zero"**In reply to:**Brian Radak: "Re: Autocorrelation function of Rg goes down without fluctuation around zero"**Next in thread:**Brian Radak: "Re: Autocorrelation function of Rg goes down without fluctuation around zero"**Reply:**Brian Radak: "Re: Autocorrelation function of Rg goes down without fluctuation around zero"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]

Usually, the autocorrelation function of Rg is used to find if the

structure is well equilibrated or not, and to find relaxation time for

them.

What do you mean with "It's also not common to use pressure control for

these kinds of calculations"? Which method do you suggest for perfotming

simulations?

Regards,

Farideh

On Aug 18, 2016 8:30 PM, "Brian Radak" <bradak_at_anl.gov> wrote:

*> I've not used either of "measure rgyr" or "g_analyze", so I can only
*

*> assume they are correctly implemented.
*

*>
*

*> I have pretty significant reservations about including your equilibration
*

*> components in the calculation. It's also not common to use pressure control
*

*> for these kinds of calculations, but I don't have a specific reason as to
*

*> why that's bad other than that it changes the physical meaning of the
*

*> result (I don't know what you want C(t) for, I assume this is not a
*

*> problem).
*

*>
*

*> My guess is that you just have very poor statistics due to a long
*

*> characteristic time. This is not unexpected for a (presumably large) system
*

*> like a dendrimer.
*

*>
*

*> Cheers,
*

*>
*

*> Brian
*

*>
*

*> On 08/18/2016 10:31 AM, faride badalkhani wrote:
*

*>
*

*> This system is a Dendrimer, which is a hyperbranched polymer. I used the
*

*> following script to calculate the radius of gyration:
*

*>
*

*> set outfile [open rg.dat w]
*

*> set mol [molinfo top]
*

*> set sel [atomselect top "all not waters"]
*

*> set frames [molinfo $mol get numframes]
*

*> for {set i 0} {$i < $frames} {incr i} {
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*> $sel frame $i
*

*> $sel update
*

*> puts $outfile "$i [measure rgyr $sel]"
*

*> }
*

*> close $outfile
*

*> $sel delete
*

*>
*

*> For calculating the C(t) I used the g_analyze in gromacs and the Rg
*

*> computed using the above script. for plotting the data I used the whole
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*> simulation time (40 ns). I loaded all the trajectory files in VMD and then
*

*> calculated the Rg.
*

*>
*

*> Regards,
*

*> Farideh
*

*>
*

*> On Thu, Aug 18, 2016 at 7:51 PM, Brian Radak <bradak_at_anl.gov> wrote:
*

*>
*

*>> What system is this? How did you compute Rg and C(t)? Which parts of the
*

*>> trajectory were used?
*

*>>
*

*>> Correlation functions can be very slow to converge, 5 ns might not be
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*>> enough.
*

*>>
*

*>> On 08/18/2016 10:16 AM, faride badalkhani wrote:
*

*>>
*

*>> 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:
*

*>>
*

*>> https://www.dropbox.com/sh/mcnkxt9pefnqgrz/AADRRw4-dJEEx0SBlU2Fuekma?dl=0
*

*>>
*

*>> p.s. Bond distances, angles, dihedrals and impropers were taken form
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*>> CHARMM GENERAL FORCE FIELD and the charges were assigned using CGen FF on
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*>> paramchem.
*

*>>
*

*>> Thank you so much for your help!
*

*>> Regards,
*

*>> Farideh
*

*>>
*

*>> On Thu, Aug 18, 2016 at 7:17 PM, Brian Radak <bradak_at_anl.gov> 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.
*

*>>>
*

*>>> HTH,
*

*>>> Brian
*

*>>>
*

*>>>
*

*>>> 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.
*

*>>>>
*

*>>>> Regards,
*

*>>>> Farideh
*

*>>>>
*

*>>>
*

*>>> --
*

*>>> Brian Radak
*

*>>> Postdoctoral Appointee
*

*>>> Leadership Computing Facility
*

*>>> Argonne National Laboratory
*

*>>>
*

*>>> 9700 South Cass Avenue, Bldg. 240
*

*>>> Argonne, IL 60439-4854
*

*>>> (630) 252-8643
*

*>>> brian.radak_at_anl.gov
*

*>>>
*

*>>
*

*>>
*

*>> --
*

*>> Brian Radak
*

*>> Postdoctoral Appointee
*

*>> Leadership Computing Facility
*

*>> Argonne National Laboratory
*

*>>
*

*>> 9700 South Cass Avenue, Bldg. 240
*

*>> Argonne, IL 60439-4854
*

*>> (630) 252-8643
*

*>> brian.radak_at_anl.gov
*

*>>
*

*>
*

*>
*

*> --
*

*> Brian Radak
*

*> Postdoctoral Appointee
*

*> Leadership Computing Facility
*

*> Argonne National Laboratory
*

*>
*

*> 9700 South Cass Avenue, Bldg. 240
*

*> Argonne, IL 60439-4854
*

*> (630) 252-8643
*

*> brian.radak_at_anl.gov
*

*>
*

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