Can you give me some more details?<br>
Are the ions just in bulk water?<br>
Or are the ions going from bulk water through some protein and coming to bulk water?<br>
If you can explain me the situation, then people here can help you better.<br>
I recently did some work in these lines too.<br>
<br><br><div><span class="gmail_quote">On 2/23/06, <b class="gmail_sendername">Alexandre Suman de Araujo</b> <<a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
My calculations is for ions in water and I´m using the mass of the ions.<br>My units are correct, a think this is not my problem.<br>Thank´s<br><br>Alexandre Suman de Araujo<br><a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br
</a><br>UIN: 6194055<br>IFSC - USP - São Carlos - Brasil<br><br><br><br>Viswanadham Sridhara wrote:<br><br>> IF it is for pure water (bulk), then mass of the water should be<br>> m=(18e-3)/Ava, where Ava is the Avogadro number.
<br>> KbT/m works out to be 1.3857e+005;<br>><br>> On 2/22/06, * Alexandre Suman de Araujo* <<a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a><br>> <mailto:<a href="mailto:asaraujo@if.sc.usp.br">
asaraujo@if.sc.usp.br</a>>> wrote:<br>><br>> Yes, I tried and I had bad values too.<br>> I think the Kubo equation is to use in the non-normalized VAC,<br>> since the<br>> D = kT/M*integral is for normalized ones.
<br>><br>> Alexandre Suman de Araujo<br>> <a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a> <mailto:<a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a>><br>> UIN: 6194055
<br>> IFSC - USP - São Carlos - Brasil<br>><br>><br>><br>> Viswanadham Sridhara wrote:<br>><br>> > Did you try 1/3*your integral<br>> ><br>> > On 2/22/06, *Alexandre Suman de Araujo* <
<a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a><br>> <mailto:<a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a>><br>> > <mailto:<a href="mailto:asaraujo@if.sc.usp.br">
asaraujo@if.sc.usp.br</a> <mailto:<a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a>>>><br>> wrote:<br>> ><br>> > Hi Gmxers.<br>> ><br>> > I´m trying to calculate coefficient diffusion values for
<br>> ions in<br>> > water.<br>> ><br>> > I made some calculations using MSD and the results were in good<br>> > agreement with experimental values.<br>> >
<br>> > The problem is when I try to calculate it from integration<br>> of VAC<br>> > function for the ions, like in Rasaiah paper (J. Chem Phys.<br>> 101(8)<br>> > 6964).
<br>> ><br>> > I made 40 simulations of 250 ps with 1 fs time step and<br>> output of<br>> > velocities every 1 fs. >From these simulations I calculated the<br>> > normalized VAC function from these 40 simulations and
<br>> calculated the<br>> > mean. This methodology is implemented to have a good<br>> statistic and a<br>> > more smooth VAC function (without fluctuations that could<br>> make the
<br>> > integral diverge). With this VAC function I use this equation to<br>> > calculate do diffusion coefficient:<br>> ><br>> > D = kT/M*Y, where Y is the integral of the VAC function
<br>> ><br>> > The problem is that the value calculated from VAC does not<br>> agree with<br>> > one calculated from MSD.<br>> ><br>> > Does anyone knows any trick to perform this kind of calculation?
<br>> > In the<br>> > paper cited above these two values are very near and I´m<br>> using the<br>> > same<br>> > methodology.<br>> ><br>> > Thank´s!
<br>> ><br>> > --<br>> > Alexandre Suman de Araujo<br>> > <a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a> <mailto:<a href="mailto:asaraujo@if.sc.usp.br">
asaraujo@if.sc.usp.br</a>><br>> <mailto: <a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a> <mailto:<a href="mailto:asaraujo@if.sc.usp.br">asaraujo@if.sc.usp.br</a>>><br>> > UIN: 6194055
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</a><br></blockquote></div><br><br clear="all"><br>-- <br>Viswanadham Sridhara,<br>Graduate Research Assistant,<br>Old Dominion University, "VIRGINIA".<br>