# [gmx-users] rdf doesn't tend to unity

Ester Chiessi ester.chiessi at uniroma2.it
Tue Sep 13 18:13:58 CEST 2005

```----- Original Message -----
From: "David" <spoel at xray.bmc.uu.se>
To: "Discussion list for GROMACS users" <gmx-users at gromacs.org>
Sent: Tuesday, September 13, 2005 4:36 PM
Subject: Re: [gmx-users] rdf doesn't tend to unity

>
>> >> >> I calculated the MD of a polymeric network in SPC water. I was very
>> >> >> surprised when I found that the rdf between the oxygen atoms of the
>> >> >> polymer
>> >> >> and the water oxygen atoms doesn't reach the value of 1 at high r
>> >> >> values.
>> >> >> The g(r) remains lower than unity and tends to 0.87.
>> >> >> I could expect a value higher than 1, eventually for inhomogeneity,
>> >> >> but
>> >> >> not
>> >> >> lower.
>> >> > Test whether the integral (4 pi r^2 g(r)) goes to #Ow
>> >> The number of polymer oxygens is 611.
>> >> OW number is 8773.
>> >> The box is cubic with size of 6.707 nm.
>> >> The integral in the 0-3.3535 nm distance interval results 136.16 nm^3.
>> > It should go to 8773, did you include the terms above in the integral?
>>
>> I calculated the integral of    (4 pi r^2 g(r))  with r from 0 to 3.3535
>> nm
>> and the result was 136.16 nm^3.
>>
>> If you multiply this value for #OW and divide for the box volume, you
>> obtain
>> a number of oxygens (3959)  lower than that you can estimate (4593) by
>> considering a sphere of water with radius 3.3535 and density equal to the
>> water average density in the box.
>>
>> The ratio 3959/4593 is 0.86, the limiting value of g(r). If it is not a
>> chance, what could it mean?
> It's correct. Do you have a cluster of polymer surrounded by water?

Some polymer regions are aggregated, some others more extended.
Ester

> --
> David.
> ________________________________________________________________________
> David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
> Dept. of Cell and Molecular Biology, Uppsala University.
> Husargatan 3, Box 596,          75124 Uppsala, Sweden
> phone:  46 18 471 4205          fax: 46 18 511 755
> spoel at xray.bmc.uu.se    spoel at gromacs.org   http://xray.bmc.uu.se/~spoel
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```