[gmx-users] Dispersion correction in a heterogeneous system

[Berk Hess]

Hi,

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.

Berk

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
















Dear gmx-users,

 

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,

Mikhail

 

=============================================

Dr Mikhail Stukan

Schlumberger Dhahran Carbonate Research Center,

Dhahran Techno Valley  - KFUPM,

P.O. Box 39011, Dammam / Doha Camp  31942,

Kingdom of Saudi Arabia

Tel: +966 3 331 0300 ext 6182

Fax:+966 3 330 0845

mstukan at slb.com

 







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