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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: Georg Heinze <georg.heinze@...6617...>
Date: Thu, 06 Jul 2017 15:01:15 +0200

Dear Ray,

thank you for your reply. So what you are saying is that fix qeq/reax is actually not QEq but EEM, just as in the Reax Code, correct? Also I found an old correspondence between you and Suleiman Oloriegbe. There a tapering function for the coulombic interaction term was mentioned. Is this function controlled by the tapering parameter of fix qeq/reax and am I correct in assuming that changing this term will not change the charges in my system but rather taper the coulombic interaction between the charged atoms for long distances? Finally, do you know whether this is done the same way in LAMMPS as in the Reax Code?


Zitat von Ray Shan <rshan@...1795...>:

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 for more info.


From: Georg Heinze <georg.heinze@...6617...>
Date: Monday, July 3, 2017 at 5:18 AM
To: "" <>
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...<mailto: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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