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)

**# 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**

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 potential, 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.

axel.