[lammps-users] Quartic and class2 angle styles term differences

EmersonPL · January 25, 2021, 3:18pm

Dear colleagues,

Sorry in advance, but I ended up with a very simple question. I just want to confirm that I understand correctly what I read in the Lammps manual.

Suppose I fitted an energy curve for angle bending term from quantum mechanics data and the expression I got was:

In the Lammps manual for the “quartic angle style”, it is described that a factor of 1/2 is included in K. Due to the parameters I obtained when adjusting the curve, I would have to use K1 = 50, K2 = -100 and K = 150, correct?

Another possibility would be to use the “class2 angle style” (with M and N = 0). However, in this case (as it is not explicitly stated) I would not consider a factor of 1/2 in K. So I would normally use K1 = 100, K2 = -200 and K = 300, correct?

Thanks for a while.

Best regards,

Emerson Parazzi Lyra

PhD candidate

School of Chemical Engineering

University of Campinas
500 Albert Einstein Ave, Campinas, SP, 13083-852, Brazil

Email: [email protected]

Orcid ID: https://orcid.org/0000-0002-7969-3764

akohlmey · January 25, 2021, 4:34pm

The best way to confirm correctness is to set up a series of test inputs that have only 3 atoms in different conformations with only a single angle between them defined as topology, define only the angle style and the corresponding coefficients, have no fixes etc. defined, and then do a ‘run 0’ and write out a dump with the current forces.

That would allow you to confirm in detail whether the potential function(s) and the corresponding forces work as you would expect.

Axel.

EmersonPL · January 25, 2021, 4:53pm

Thank you very much for the clarification, Dr. Axel Kohlmeyer!