Inquiry on Epsilon/Sigma Units in Quartic Bonds

Dear LAMMPS Users,

Hello. This is Masato Koizumi, and I am using LAMMPS for my molecular dynamics simulations.

I would like to ask a question in regards to the quartic bond potential. If the sigma and epsilon parameters in the WCA terms of the quartic potential are set intrinsically to 1.0, I was curious if the bond_style quartic command can only be used for non-dimensional LJ units. Thank you.


Masato Koizumi

I think that’s all that it’s been used for, i.e. FENE chains.

However, I don’t see a reason we couldn’t allow sigma/epsilon to be specified
(optionally) as part of the bond coeff command. In which case you would
have to carefully choose all the other params, as it describes in the doc

When monomers with sigma = epsilon = 1.0 are used, the following choice of parameters gives a quartic potential that looks nearly like the FENE potential: K = 1200, B1 = -0.55, B2 = 0.25, Rc = 1.3, and U0 = 34.6878. Different parameters can be specified using the bond_coeff command, but you will need to choose them carefully so they form a suitable bond potential.

to end with something that worked.