# Questions about compute centro/atom command

(1) In the fifth paragraph of the doc page, it says that “An atom at a surface will have a large positive parameter. If the atom does not have N neighbors (within the potential cutoff), then its centro-symmetry parameter is set to 0.0.”. I understand the first sentence. But I am confused that if an atom is on the surface, it will not have N neighbors. And the centro-symmetry parameter is set to 0.0? Is this contrary to the first sentence?
(2) It says that “Only atoms within the cutoff of the pairwise neighbor list are considered as possible neighbors.” in the sixth paragraph. Does this mean the cutoff of the pairwise neighbor list must be greater than the first-nearest neighbor distance?
(3) The recommended values of this parameter are listed in the doc page. But it is from a nanoindentation simulation of gold. I think this parameter is only sensitive to the material (constant lattice distance and lattice structure). I am wondering if I want to use this parameter in analysis of radiation damage of gold, is that proper to use the same values of this parameter? And if I want to use this parameter to analyze the simulation results of copper, the values should be changed. Is that right?
Any help would be much appreciated! Thanks very much!

With My Best Regards
Liu

help me?

(1) In the fifth paragraph of the doc page, it says that “An atom at a
surface will have a large positive parameter. If the atom does not have N
neighbors (within the potential cutoff), then its centro-symmetry parameter
is set to 0.0.”. I understand the first sentence. But I am confused that if
an atom is on the surface, it will not have N neighbors. And the
centro-symmetry parameter is set to 0.0? Is this contrary to the first
sentence?

​no.​

​an atom at the surface *still* can have N nearest neighbors, only they all
on the same (bulk) side and thus some are quite further away and thus
leading to the larger value. only under extreme circumstances, there will
be not enough atoms. i suggest you create some simple example structures
and ​validate that the compute is giving the values that it claims it would.

(2) It says that “Only atoms within the cutoff of the pairwise neighbor
list are considered as possible neighbors.” in the sixth paragraph. Does
this mean the cutoff of the pairwise neighbor list must be greater than the
first-nearest neighbor distance?

​it better be, or else your atoms would not interact.​

(3) The recommended values of this parameter are listed in the doc page.
But it is from a nanoindentation simulation of gold. I think this parameter
is only sensitive to the material (constant lattice distance and lattice
structure). I am wondering if I want to use this parameter in analysis of
radiation damage of gold, is that proper to use the same values of this
parameter? And if I want to use this parameter to analyze the simulation
results of copper, the values should be changed. Is that right?

​what is provided in the documentation is an *example*. how to interpret
the result of a computation is not really a LAMMPS question and thus you
reference given in the documentation and take it from there.

axel.​

There is also a difference between the cutoff (which you specify

with the pair style command)

and nearest neighbors. If your cutoff is long enough

to include more than nearest neighbors, then as Axel

said, even at the surf you can have enough atoms

within the cutoff to calc the CS param, and it will

be large.

Steve