Seeking Information regarding Simulation of DWCNT buckling in LAMMPS 20110407 version

Hi All
I am using LAMMPS to calculate the critical buckling load for DWCNTs. In order to validate my LAMMPS code I used it to calculate the critical load values for a few double walled
carbon nanotubes and compared it with results published in journal articles. I am new to Molecular Dynamics simulation and have no clue where I went wrong. In some of the literature an older version of LAMMPS is used (some version in year 2010), and when I compare the results with my simulation I got critical load values underestimated by ~15%.
I am using the version on LAMMPS published on 7th of April 2011. My input file and data files are given below. I would be very obliged if someone can answer the following queries:

1. Is it expected that in a newer version of LAMMPS results will be different.

2. Is there anything wrong in my input file.

I briefly summarize the problem:

Inner tube type = armchair, chirality = 10
Outer tube type = armchair, chirality = 10
Total length = 72.3001 A
Free length = 60.0458 A (Length excluding the constrained atoms)
Type of potential used = AIREBO
Velocity of pushing the nanotube = 0.05 A/ps
Simulation temperature = 300 K

step 1: I am minimizing the system
step 2: Then I perform nvt analysis to compress the nanotube and record the force displacement

========LAMMPS INPUT FILE===============================================