[gmx-users] Dispersion correction in a heterogeneous
mstukan at slb.com
Mon Oct 4 10:44:12 CEST 2010
Thank you very much for the answer. It was not clear from my post (sorry for that) but my main concern was indeed related to the inhomogeneity near the surfaces. I am not targeting complete quantitative agreement, but I would like to be save from "hidden" undesired qualitative effects. As far as I know, the dispersion correction contributes something like 2-5% to the surface tension value (depending on the cut-off). If I consider different salt concentrations then the inhomogeneity near the surface will be of different order (compare to the bulk). So taking into account your recommendation, long cut-off with no dispersion correction looks like a better option.
Thanks and regards,
>Date: Mon, 4 Oct 2010 10:16:35 +0200
>From: Berk Hess <gmx3 at hotmail.com>
>Subject: RE: [gmx-users] Dispersion correction in a heterogeneous
>To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>Message-ID: <COL113-W10EA30F423207830F450CE8E6C0 at phx.gbl>
>Content-Type: text/plain; charset="iso-8859-1"
>I think you are looking at the wrong issue.
>Unless your concentration is ridiculously high, the dispersion heterogeneity will be irrelevant.
>Furthermore, at distances were the correction works, the distribution will be close to homogenous.
>But you do have an issue with dispersion correction because of strong spatial inhomogeneity because
>of the interface. The dispersion correction will have no direct effect on the surface tension.
>What is the best way to proceed depends very much on the questions you want to answer.
>The surface tension of many water models is off by 30%, so you probably won't get quantitatively
>correct results anyhow. (note that also many "standard" combinations of Na and Cl parameters are very bad)
>If you want an accurate number, given the force field limitations, you need to use a long LJ cut-off,
>possibly with twin-range settings for efficiency. But force fields often have not been parametrized with this.
>And finally, ion C6 parameters do not give realistic dispersion strengths in most cases, but as I said,
>this effect will be negligible in normal concentration ranges.
>>From: mstukan at slb.com
>>To: gmx-users at gromacs.org
>>Date: Sat, 2 Oct 2010 12:40:23 +0000
>>Subject: [gmx-users] Dispersion correction in a heterogeneous system
>>Although this task has been already discussed few
>>years ago (http://lists.gromacs.org/pipermail/gmx-users/2007-January/025668.html)
>>the full summary is not clear to me. So I would really appreciate if somebody
>>could give me advice on the following subject.
>>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)?
>>Thanks you very much in advance,
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