I would like to know what precautions one should take while performing tensile testing in LAMMPS. For example; tensile testing of Copper. I would like to perform the test at room temperature conditions (300K and 1 atm pressure) hence how should we go about relaxing the structure and temperature (using fixes) so as to achieve good results? My box size is 100 x 100 x 100 Angstroms
Hello Rohit,
I think your problem already imposes the demand for simulations to be carried out at 1 atm pressure, so NPT would be required (assuming its periodic). To achieve the required pressure and temperature for long enough time such that the system pressure and temperature oscillations are very small around the desired values. During tensile deformation you can either use fix deform (with flag erate) to achieve a constant strain rate. The transverse directions can be relaxed by NPT @ 1 atm.
Caution: The simulation cell needs to be big enough that you don't have periodic boundary effect during deformation.
The issue I come across is with pressure fluctuations.After creating the box I used the velocity command to ramp the temperature to 300 K and then used NPT to minimise the energy, but after 80000 timesteps, even though the total energy minimises and converges, the pressure of the system is a large value. Also when the deformation starts the pressure is a large value at zero strain. Is this correct behaviour?
If the magnitude of pressure fluctuations is fairly large at the end of your NPT, this means that you need to run for a longer time. Secondly, if you want to have the simulation at an initial 1 atm pressure, the initial stress/pressure would be 1 atm for an equilibrated structure at 0% strain (this was your problem definition).
I would suggest you try to minimize the pressure oscillations about your desired pressure before worrying about deformation.
Hope this helps.
Ilaksh
First of all, make sure that on what kind of system you want carry out tensile deformation. Whether it is nanowire or nanopiilar or bulk specimen (they are different, the way the boundary conditions were used during the MD simulations). If the system is nanowire or nanopillar, there is exist an initial stress/pressure ( in order of GPa) even at zero strain, which is due to the free surfaces (Refer the attached paper).
Article Molecular dynamics simulation studies on the size dependent ...
Thanks Ilaksh Adlakha for the answer. Why should the pressure be varying this much even though the energy has reached a minima? the script I am runnning now shows initlal "Press" as 3503. But after 10000 steps the pressure varies in double digit figures, but randomly. Sometimes it becomes negative and sometime positive. All the while the total energy is around a minima. Should I be running for a much longer time and wait to reach 1 atm pressure or is there a way to get around this issue?
Hello Rohit,
For the better understanding of how ensembles work in MD you should read a good book or a review paper, because I think you are unclear on the way these ensembles work. My best guess, is that if the system pressure is converging towards 1 atm, but the oscillations are still large enough you should run for a much longer time.
Regards,
Ilaksh
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