[gmx-users] New ion parameters and OPLS-AA

[Reza Salari]

Thanks for your response.

While I will try that (although it seems it needs quite amount of scripting), I remember in the past some people in the mailing list mentioned problems while using nonbond_params directvie with OPLS-AA and in response it was generally suggested to avoid doing this kind of mixing. Like Dr Abraham suggestion here:

http://www.mail-archive.com/gmx-users@gromacs.org/msg23147.html

Have you tried this kind of mixing for OPLS successfully, without being overridden by the default rules?

The main thing that I am still unsure about is how the previously mentioned paper converted the sigma values for different combination rules. It seems that there must be a relatively direct way to do this without going through the re-parametrization process.

Regards,
Reza Salari



________________________________
From: Andrew Paluch <apaluch at nd.edu>
To: Discussion list for GROMACS users <gmx-users at gromacs.org>
Sent: Tue, December 15, 2009 4:34:18 PM
Subject: Re: [gmx-users] New ion parameters and OPLS-AA

Read the manual.  You can explicitly declare all of your cross terms rather than using the same mixing rule for all terms.  You can easily write a script to modify your input files accordingly,

Andrew


On Tue, Dec 15, 2009 at 4:06 PM, Reza Salari <resal81 at yahoo.com> wrote:

Hi All,
>
>Recently there has been a new set of ion parameters published by Joung and Chetham and I am interested in running some test runs using these parameters. These set of parameters are based on using LB rule (arithmetic mean) for sigmas.
>
>However I am using OPLS-AA ff so I am using the combination rule 3 (geometric mean of corresponding A and B values). My question is that can I use the exact sigma values from Cheatham for my simulations? I'm almost positive that I have to change these sigma values to be consistent with the combination rule that I am using. In fact there is a paper by Horinek et al that has a nice table of different ionic sigma and epsilon values from different parameter sets (Aqvist, Jensen, Cheatham,..). The article is
> here:
>http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JCPSA6000130000012124507000001&idtype=cvips&gifs=Yes
>
>In that table, they have mentioned two sigmas; a usual sigma (which is used with rule 2) and a sigma prime (which can be used with rule 3). However it seems sort of unclear to me how they got these value since in some references that they've mentioned I could find either sigma or sigma prime, not both. So I am guessing there must be some way to convert these two sigmas to each other.
>
>So does anyone know if there is such way? Does GROMACS internally treats sigmas as "sigma prime" for OPLS-AA? I looked at the manual and also searched the mailing list to find an explanation but without luck. I really appreciate any help on
> clarifying this.
>
>Regards,
>Reza Salari
>
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