Did you try 1/3*your integral<br><br><div><span class="gmail_quote">On 2/22/06, <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;">Hi Gmxers.<br><br>I´m trying to calculate coefficient diffusion values for ions in 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 of VAC<br>function for the ions, like in Rasaiah paper (J. Chem Phys. 101(8) 6964).
<br><br>I made 40 simulations of 250 ps with 1 fs time step and output of<br>velocities every 1 fs. From these simulations I calculated the<br>normalized VAC function from these 40 simulations and calculated the<br>mean. This methodology is implemented to have a good statistic and a
<br>more smooth VAC function (without fluctuations that could 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 agree with<br>one calculated from MSD.<br><br>Does anyone knows any trick to perform this kind of calculation? In the<br>paper cited above these two values are very near and I´m using the 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><br>UIN: 6194055<br>IFSC - USP - São Carlos - Brasil<br><br>_______________________________________________
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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>