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Re: [lammps-users] Regarding two Tersoff potentials
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Re: [lammps-users] Regarding two Tersoff potentials

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Sat, 16 Sep 2017 09:55:47 -0400

On Sat, Sep 16, 2017 at 4:20 AM, Sharma MD <> wrote:
Hello all,
I am using hybrid pair style under which i am using two tersoff potentials. With first tersoff i am taking the interaction between Be, Be and C and the second tersoff potential is used for C. I assume the C-C interactions with first tersoff are replaced by second tersoff potential. I want to know is it right way to assign C-C interaction by second tersoff.(The program is running without any error)

as has been explained *many* times on this mailing list, ​tersoff is not a pairwise additive po​tential, so you do not only need to consider interactions between pairs of elements (i.e. Be-Be, Be-C, C-C) but also you have terms for triples. with your setup, you will have an inconsistent representation: how would terms of the kind C-C-Be or Be-C-C or C-Be-C be represented? the neighbor list for the first tersoff potential will not contain the C-C pairs, while the second tersoff potential will not know about pairs containing Be atoms. so the only way to implement this kind of mix of parameters is to merge the various parameter terms from the two potential files into a single potential file, i.e. replace terms in BeC.abop.tersoff with terms from C.abop.tersoff.

however, it is considered a bad idea to mix terms for the same potential function from different parameterizations. each parameterization can follow a different strategy to balance the components contributing to the interactions. after all you are reducing something where everything interacts with everything (e.g. as represented in a quantum mechanical hamiltonian) with a simplified model via some kind of empirical mean-field approach. this does not only apply to tersoff, but to all potentials implemented in LAMMPS. if you mix contributions from different parameterizations, the result has to be considered a new parameterization and thus first needs to be characterized in the same way with sufficient detail. 



# There are three types of atoms Cu, Be and C. Cu:1-3, Be-4, C:5-7.
pair_style hybrid tersoff tersoff morse 2.5 eam

pair_coeff * * tersoff 1 BeC.abop.tersoff NULL NULL NULL Be C C C
pair_coeff * * tersoff 2 C.abop.tersoff NULL NULL NULL NULL C C C
pair_coeff 1*3 5*7 morse 0.087 5.14 2.05          
pair_coeff 1*3 1*3 eam Cu_u3.eam
pair_coeff 1*3 4 morse  2.14 5 2.065

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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.