About reaxff force field with no chemical reaction

Hello guys,
        I have a system which consists of small gases such as hydrogen,
carbon monoxide, carbon dioxide and water. For water, I used TIP4P/2005
model. For gases, fortunately, I found that there is a potential named
ffield.reax.cho in the potentials folder.
        Since I haven't use reaxff before, and my system doesn't require
chemical reaction because I just want to investigate the diffusion of
this system.
        My question is that can I use pair_style reax/c to define the
interactions between these gases with ffield.reax.cho potential and use
fix nvt, nve to integrate velocity and position rather than fix qeq/reax?

​technically, yes. but it is a very bad idea in general, and in your case,
it is an *extremely* bad idea.​ when doing a simulation with reaxff you
should almost always describe everything with reaxff. in addition, there is
the performance issue. reaxff is massively slower than a conventional pair
style, like several hundred times. so you would be wasting time.

        If I can, then how can I define the interactions between gases
and water? If I use lj/cut/coul/long, how can I get the epsilon and sigma
for L-J parameters because there is no epsilon and sigma in ffield.reax.cho?

​this is just one of the ​problems and, if you have the answer to this
question, you don't really need reaxff *anyway*.
so the right way to proceed is to do a proper and more thorough search of
the published literature and search for publications, that describe the
interactions between water and the gases you are looking to investigate.
since for any realistic system, the concentration of those gasses will be
low, you should not worry much about the interaction between the gas
molecules themselves. but you must worry about having all of them
parameterized for the same water model and for the same conditions
(temperature, pressure). as a measure of last resort, you can look into
generalized force field versions of amber or charmm. mind you, even that
will likely be more meaningful that what you are proposing. careful testing
and validation, of course, is a must for any attempt to simulate such a
complex system.