Absurd Stress Values for Nanowires Under Periodic Boundary

Dear All,

I am trying to investigate the effect of boundary condition on mechanical properties of nanowires. I have investigated a silver nanowire under non-periodic boundary condition which is giving me proper stress-strain curve and appropriate stress values. But while I am investigating the effect of periodic boundary condition (same program of non-periodic boundary condition with boundary condition changed only), it is giving a strangely fluctuating stress-strain curve with abruptly high stress values. The same is happening for an aluminium nanowire also.

Kindly give your suggestions regarding this problem. I am attaching the input file, potential file and the log file (containing load and displacement values) of my periodic run for silver nanowire. I am using LAMMPS 2016 version (64 bit). Thank you all in advance.
Ag.eam.alloy.txt
Ag_deform.txt
log.NW_AG_DEFORM

Hi Jit,

If you are simulating a nanowire, there is no point to use periodic boundary conditions (pbcs) in the transverse directions. In my opinion, one only needs to use artificial pbcs for non-periodic systems (such as wires and films) when one wants to use some features requiring pbcs, such as ewald summation for coulomb potential. In this case, one has to make sure there is large enough vacuum layer.

Bruce

Jit Sarkar <[email protected]…24…> 于 2018年12月14日周五 17:26写道:

Dear All,

I am trying to investigate the effect of boundary condition on mechanical properties of nanowires. I have investigated a silver nanowire under non-periodic boundary condition which is giving me proper stress-strain curve and appropriate stress values. But while I am investigating the effect of periodic boundary condition (same program of non-periodic boundary condition with boundary condition changed only), it is giving a strangely fluctuating stress-strain curve with abruptly high stress values. The same is happening for an aluminium nanowire also.

yes, because you are moving atoms on top of each other due to the periodic boundaries.
the proper way to deform a periodic wire is by deforming the box, not by pushing atoms around.
…and as bruce mentioned, you should not use PBC orthogonal to the periodic dimension, or you have to leave sufficient vacuum so that atoms do not interact across PBC (except for the peridiodic dimension of the effectively infinite nanowire).

axel.

Dear Sir,

Thank you very much for your kind help. Your information and suggestions proved very helpful to me.