Is it necessary to define pair_coeff for all atom pairs in the Soft Potential?

Please post to the list, not to me.

Mixing occurs if you don’t specify a I/J
interaction when I != J. I.e. if you
specify the 1/1 and 2/2 interactions,
that sentence is telling you what the

1/2 interactions will be. See the pair_modify
doc page for more details on mixing.