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Hi,<br><br>I think you are looking at the wrong issue.<br>Unless your concentration is ridiculously high, the dispersion heterogeneity will be irrelevant.<br>Furthermore, at distances were the correction works, the distribution will be close to homogenous.<br>But you do have an issue with dispersion correction because of strong spatial inhomogeneity because<br>of the interface. The dispersion correction will have no direct effect on the surface tension.<br><br>What is the best way to proceed depends very much on the questions you want to answer.<br>The surface tension of many water models is off by 30%, so you probably won't get quantitatively<br>correct results anyhow. (note that also many "standard" combinations of Na and Cl parameters are very bad)<br>If you want an accurate number, given the force field limitations, you need to use a long LJ cut-off,<br>possibly with twin-range settings for efficiency. But force fields often have not been parametrized with this.<br>And finally, ion C6 parameters do not give realistic dispersion strengths in most cases, but as I said,<br>this effect will be negligible in normal concentration ranges.<br><br>Berk<br><br><hr id="stopSpelling">From: mstukan@slb.com<br>To: gmx-users@gromacs.org<br>Date: Sat, 2 Oct 2010 12:40:23 +0000<br>Subject: [gmx-users] Dispersion correction in a heterogeneous system<br><br>
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<p class="ecxMsoNormal">Dear gmx-users,</p>
<p class="ecxMsoNormal"> </p>
<p class="ecxMsoNormal">Although this task has been already discussed few
years ago (<a href="http://lists.gromacs.org/pipermail/gmx-users/2007-January/025668.html" target="_blank">http://lists.gromacs.org/pipermail/gmx-users/2007-January/025668.html</a>)
the full summary is not clear to me. So I would really appreciate if somebody
could give me advice on the following subject. </p>
<p class="ecxMsoNormal"> </p>
<p class="ecxMsoNormal">I am trying to simulate an aqueous solution of NaCl. One of
the properties I am interested in is surface tension, which means that
correction to the pressure could be quite important. But the system is
heterogeneous: values of C_6 differs from 0.0003 (for Na-Na) to 0.0180 (for
Cl-Cl), so they are not comparable. Is it worth to use dispersion correction in
such a system? Or such results will have no real meaning and the proper way would
be to switch off the dispersion correction and just increase cut-off (or
perform rerun with higher cut-off)? </p>
<p class="ecxMsoNormal"> </p>
<p class="ecxMsoNormal">Thanks you very much in advance,</p>
<p class="ecxMsoNormal">Mikhail</p>
<p class="ecxMsoNormal"> </p>
<p class="ecxMsoNormal"><span style="font-family: 'Arial','sans-serif'; font-weight: normal;" lang="EN-GB">=============================================</span></p>
<p class="ecxMsoNormal"><span style="font-family: 'Arial','sans-serif'; font-weight: normal;" lang="EN-GB">Dr Mikhail Stukan</span></p>
<p class="ecxMsoNormal"><span style="font-family: 'Arial','sans-serif'; font-weight: normal;">Schlumberger Dhahran Carbonate Research Center,</span></p>
<p class="ecxMsoNormal"><span style="font-family: 'Arial','sans-serif'; font-weight: normal;">Dhahran Techno Valley - KFUPM,</span></p>
<p class="ecxMsoNormal"><span style="font-family: 'Arial','sans-serif'; font-weight: normal;">P.O. Box 39011, Dammam / Doha Camp 31942,</span></p>
<p class="ecxMsoNormal"><span style="font-family: 'Arial','sans-serif'; font-weight: normal;">Kingdom of Saudi Arabia</span><span style="font-family: 'Arial','sans-serif'; font-weight: normal;"></span></p>
<p class="ecxMsoNormal"><span style="font-family: 'Arial','sans-serif'; font-weight: normal;" lang="EN-GB">Tel: +966 3 331 0300 ext 6182</span></p>
<p class="ecxMsoNormal"><span style="font-family: 'Arial','sans-serif'; font-weight: normal;" lang="EN-GB">Fax:+966 3 330 0845<br>
<a href="mailto:mstukan@slb.com"><span style="color: blue;">mstukan@slb.com</span></a></span></p>
<p class="ecxMsoNormal"> </p>
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