I have a question for the potential form of the combination of ZBL and Tersoff in the LAMMPS script. The potential form is:
V_ij = (1 - f_F(r_ij))*V_ij(ZBL) + f_F(r_ij)V_ij(Tersoff), and f_F(r_ij) = 1/(1+exp(-A_F(r_ij-r_C))).
There is a parameter A_F in f_F(r_ij) form to tune the ZBL effect in the combined potential. Any suggestions to choose a reasonably good value for that parameter? Is this parameter materials dependent? I am working on Copper at the moment.
The reciprocal of A_F defines the lengthscale over which the switching
occurs. Clearly, this lengthscale should lie in the range 0.05-0.5 A.
Anything greater than this will cause the ZBL core (whose range is ~ 1
A) to be weakened. Anything less than this will start to look like a
discontinuous force. A good way to calibrate more tightly would be to
start at A_F = 1/0.5 A = 2 A^-1, run MD for your application (sampling
the ZBL core) and measure energy conservation with a "natural"
timestep. Then find out how large you can make A_F before energy
conservation starts to degrade. That is the value you should use. I
notice that in potentials/SiC.tersoff.zbl, the value is 14.