COMB3 - vdwflag parameter

Dear all,

I have a very specific question concerning the implementation of COMB3 in lammps, but figured that it might interest other lammps users. “vdwflag” is one of the parameters contained in ffield.comb3, which is parsed with pair_comb3.cpp. It is equal to 0, 1, 2 or 4 and I believe is used as a switch to determine cutoffs for cubic spline functions which are supposed to terminate smoothly the vdw interaction.

My question is: in pair_comb3.cpp, the only conditions on vdw_flag are {vdwflag > 0 && vdwflag != 1}, or vdwflag==1. This means that triplets with vdwflag==2 and triplets with vdwflag==4 are treated the same. Are these two different values of the parameter only used to differentiate the nature of the triplets?

Below are the different values of vdwflag according to the triplets:

C - C - x
C - H - x
H - C - x
H - H - x

C - Cu - x
Cu - C - x
H - Cu - x
Cu - H - x

Cu - Cu - x

else vdwflag=0


Although they are treated the same now, they can and may be different in the future.


Thanks for the quick answer.