How to decide on bond coefficient values?

Hello All,

I have started using LAMMPS and I am trying to write my input file using DPD model. I need to use the bond_coeff command to define the bond coefficient for Carbon Carbon in lj units. Due to LAMMPS documentation if I have the bond length of C-C I should be able to get the unitless (lj) value by deviding it by sigma.

https://docs.lammps.org/units.html

But I could not find the value of sigma for carbon. Do you know about possible sources for sigma and epsilon values for different material? specifically for Carbon-Carbon do you know what is the bond coeff value in lj units?

If you want to model a specific material, you usually don’t use lj units. The whole point of reduced units is that they are abstract and represent just ratios between different particles.

When you want to model “real” materials, you would use “real” parameters created for your specific application that represent its properties as close as possible with the kind of model in use. Usually those can be looked up in the published literature.

Please also note that the choice of parameters and how parameters are derived are questions that are independent from the simulation program and thus this is not really a LAMMPS topic. This is something you can learn from suitable text books, lectures, publications and so on.

Thank you, but I need to use lj units because I am using DPD method and I am building up on previous works done by others using the same lj unit.

Then use what those others were using. Again, how to do your science is not a LAMMPS question.
You need to discuss these things with your adviser/supervisor/tutor etc. A forum is no substitute for that.