Tensile test with non-reactive force fields.

Hi Users,

Lammps Version: Jan/2014 and Mar/2014

I have a question about the tensile test. I am using non-reactive force fields (Amber, GAFF, CVFF, PCFF, etc). Without using “fix bond/break”, these force fields do not allow bond breaking even at high elongation. The “fix bond/break” needs special bond setting (lj 0 1 1 coul 0 1 1) at least for those versions I use. Since all pair wise interactions between 1-2 atom pairs are turned off with the special bond setting during a deformation-Tensile loading, the pressure quantity would not be correct due to no interactions of all pair atoms bonded to each other.

Finally, I would like to break a bond with certain criteria. For example, if a bond length is twice longer than the equilibrium bond length during tensile deformation, the bond needs to be broken. The reason why I am considering bond breaking with non-reactive force fields is because the pressure exponentially increase at high elongation and the simulation dose not show a fracture. In order to do this kind of simulation (with non-reactive force fields and bond breaking), do I need to customize fix command (e.g., fix bond/break or something else)?

Any comments are appreciate.


Hi James,

Turning off important parts of your atomistic forcefield is probably not a good option. I think you would need to extend fix bond break to support your FF’s requirements for special bonds. Also if you’re using fix bond break for an atomistic FF, you might need a way to deal with large forces if a hard non-bond potential like LJ is turned on between the two atoms after a bond breaks. The distance to break (Rmax) can already be set to any value, including twice the equil distance.