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Re: [lammps-users] Transition from EEM to QEq when implementing ReaxFF in LAMMPS
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Re: [lammps-users] Transition from EEM to QEq when implementing ReaxFF in LAMMPS


From: Ray Shan <rshan@...1795...>
Date: Wed, 5 Jul 2017 16:51:47 +0000

Dear Georg,

 

The original QEq proposed by Rappe and Goddard uses the slater type orbital to describe charge-charge interactions (resulted from orbital overlap), but ReaxFF uses the slightly modified approach that describes charge-charge interactions via a shielded Coulomb (the EEM approach).  The EEM approach for ReaxFF is performed via fix qeq/reax.

 

If you want to use Slater type orbital for charge-charge interactions, there is a fix qeq/slater that you can use.  You might need to find the necessary parameters from the Literature or even “guess” them.  Please see http://lammps.sandia.gov/doc/fix_qeq.html for more info.

 

Ray

 

From: Georg Heinze <georg.heinze@...6617...>
Date: Monday, July 3, 2017 at 5:18 AM
To: "lammps-users@lists.sourceforge.net" <lammps-users@lists.sourceforge.net>
Subject: Re: [lammps-users] Transition from EEM to QEq when implementing ReaxFF in LAMMPS

 

Dear all,

 

I am still wondering about the different charge equilibration methods  

in LAMMPS and the Reax Code. The main difference between my results  

and the results of Khalilov et al. is long range Coulomb repulsion  

between oxygen molecules approaching a silicon nanowire and oxygen  

atoms adsorbed on the wire. This repulsion is happening in my  

simulation because of the global, instantaneous charge redistribution  

done by QEq. The results of Khalilov et al. do not seem to show the  

repulsion even though EEM also works globally and instantaneously.

 

Does anybody of you know how one can explain this difference?

 

Kind regards,

Georg Heinze

 

Zitat von Georg Heinze <georg.heinze@...6617...>:

 

Dear all,

 

I am currently simulating the oxidation of silicon nanowires using  

ReaxFF with Parameters by Buehler et al. (Phys. Rev. Lett. 96, 2006)  

in LAMMPS. My first approach is to replicate calculations by  

Khalilov et al. (Nanoscale 5, 2013) carried out using the Reax Code.  

When comparing my data to the data obtained by Khalilov et al. I  

noticed a difference that is most likely related to charges. Charge  

equalization in LAMMPS is realized via QEq while in the Reax Code it  

is realized via EEM. In the original QEq paper by Rappe and Goddard  

(J. Phys. Chem. 95, 1991) it is mentioned that the charges obtained  

via EEM are a factor 3-6 small in comparison to charges obtained via  

QEq. In the original ReaxFF paper by van Duin et al. (J. Phys. Chem.  

A 105, 2001) it is argued that QEq and EEM are very similar and that  

the main difference between the two can be compensated by adjusting  

a shielding parameter in the Coulomb Term of ReaxFF. The question I  

have is whether and how the difference between QEq and EEM was  

considered when ReaxFF was implemented in LAMMPS.

 

Kind regards,

Georg Heinze

 

 

 

 

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