Hi Leton,
Please direct all correspondence to the LAMMPS user list, thanks.
Pair_style reax and reax/c doc pages have descriptions on how to use
pair_coeff with reax: http://lammps.sandia.gov/doc/pair_reax_c.html.
Which ffield file did you use?
If C, H , O and Ni are the 1st, 2nd, 3rd and 4th element listed in the
ffield file you are using, and your atom types are listed in the
following order you provided, then you should use 4 1 2 3 1 2. For
the 7 atom type arrangement you listed below, you should use 4 1 2 4 3
1 2.
Best,
Ray