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Re: [lammps-users] About reaxff force field with no chemical reaction
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Re: [lammps-users] About reaxff force field with no chemical reaction

From: "284237308@...1204..." <284237308@...1204...>
Date: Thu, 9 Nov 2017 15:38:41 +0800

        Thank you for the detailed explanation.  I really learned a lot about reaxff and more importantly, how to find a proper force field.

From: Axel Kohlmeyer
Date: 2017-11-08 18:39
To: 284237308@...1204...
CC: lammps-users
Subject: Re: [lammps-users] About reaxff force field with no chemical reaction

On Wed, Nov 8, 2017 at 3:00 AM, 284237308@...1204... <284237308@...1204...> wrote:
Hello guys,
        I have a system which consists of small gases such as hydrogen, carbon monoxide, carbon dioxide and water.  For water, I used TIP4P/2005 model. For gases, fortunately, I found that there is a potential named ffield.reax.cho in the potentials folder. 
        Since I haven't use reaxff before, and my system doesn't require chemical reaction because I just want to investigate the diffusion of this system. 
        My question is that can I use pair_style reax/c to define the interactions between these gases with ffield.reax.cho potential and use fix nvt, nve to integrate velocity and position rather than fix qeq/reax? 

​technically, yes. but it is a very bad idea in general, and in your case, it is an *extremely* bad idea.​ when doing a simulation with reaxff you should almost always describe everything with reaxff. in addition, there is the performance issue. reaxff is massively slower than a conventional pair style, like several hundred times. so you would be wasting time.
        If I can, then how can I define the interactions between gases and water? If I use lj/cut/coul/long, how can I get the epsilon and sigma for L-J parameters because there is no epsilon and sigma in ffield.reax.cho?

​this is just one of the ​problems and, if you have the answer to this question, you don't really need reaxff *anyway*.
so the right way to proceed is to do a proper and more thorough search of the published literature and search for publications, that describe the interactions between water and the gases you are looking to investigate. since for any realistic system, the concentration of those gasses will be low, you should not worry much about the interaction between the gas molecules themselves. but you must worry about having all of them parameterized for the same water model and for the same conditions (temperature, pressure). as a measure of last resort, you can look into generalized force field versions of amber or charmm. mind you, even that will likely be more meaningful that what you are proposing. careful testing and validation, of course, is a must for any attempt to simulate such a complex system.


        Thanks a lot for help.


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Dr. Axel Kohlmeyer  akohlmey@...12...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.