Two questions about the COMB potential

Dear all of the COMB potential users:
Recently, I have tried to simulate alloy of Cu-Si by using COMB potential. I have two questions about the potential parameters in the ffield.comb file provided in LAMMPS.
(1) The first question is about the parameters of R and D. These definition are given in the LAMMPS documentaion as follows:
R: cutoff, distance units, 0.5*(r_outer+r_inner);
D: cutoff, distance units, R-r_inner
In the ffield.comb file, the parameters for pure Cu and Si are given:
for the pure Cu, R=3.0, D=0.05
for the pure Si, R=2.90, D=0.1
My question is: how to obtain these two parameters for “Si Cu Cu” based on the parameters of pure Cu and Si?
Basically, I know that mixing rule is used, but I can’t obtain the parameters for “Si Cu Cu” provied in the ffield.comb file, where the values are R=3.05, D=0.15.
(2) The second question is: I found that all of the parameters of “Si Cu Cu” are the same as the parameters of “Si Cu Si”. I think there should be some different between them due to the three-body effect. How to explain the same parameters provided in the ffield.comb file?

Any explanations will be very appreciated.


Ray can answer this.