# General gayberne potential questions (sigma)

Hi, I am so sorry if this has already been asked or is addressed in the documentation and/or introductory materials/courses, please let me know if I should ask this elsewhere: I have a few general questions about the minimum effective particle radius (sigma) parameter for defining pair_style gayberne potentials.

• What is the physical meaning of this parameter and where does it fit into LAMMPS’ simulation of particles interacting based on Gay-Berne potentials?
• The documentation recommends that we set the minimum effective particle radius to the minimum of the three shape diameters of the particles involved in an i,i interaction. Why is this practice recommended?
• More on the documentation’s recommendation, should this recommended practice still be followed when describing potentials between dissimilar particles - in other words, is it also recommended to set the minimum effective particle radius to the minimum of the three shape diameters of either atom type of the particles? (Ex: for describing a potential between Species A with shape diameters 3 ru, 4 ru, 6 ru and Species B with shape diameters 2 ru, 8 ru, 7 ru, I should set sigma = 2 ru?)

Of course, if the answers to any of these questions are “we don’t know what your simulation needs, try things on your own and see”, please let me know. Thank you very much for your time!

Best,

springboy

The ellipsoids axes determine the shape of the ellipsoid, while the sigmac parameter is the contact distance between two ellipsoids. For small sigmac values, the minimum of the Gay-Berne potential is close to the ellipsoid axes. For increasing large values of sigmac, the minimum of the potential shifts to higher values and the attractive curve becomes more shallow. Here is a demonstration with the following parameters:

eps0 = 5.5
sigmax = 7.6
sigmay = 6.2
sigmaz = 4.3
epsx = 0.2
epsy = 0.4
epsz = 1
upsilon=1

The choice of sigmac affects pretty much the stickiness of the particle, as you can see from the shape of the potential. In this paper, I have used a reference potential computed on atomistic models to fit the Gay-Berne parameters. In that case, sigmac is just another free parameter controlling the shape of the potential.

Thank you so much hothello! The reference, visual, and explanation were extremely helpful and clarifying! Appreciate the time you gave for this!