Please keep the email in the user list.
If you are using NVT, volume most definitely will not fluctuate, no
matter which pair_style you use. Besides, How do you know volume is
fluctuating? Your "thermo_style custom" command is after your "run"
command, so your thermo output does not even contain info on volume.
Other problems with your input script.
1. timestep 1.0 is way too big.
2. Make sure you are using "ffield.reax.sio 1 3" correct, i.e.
mapping to the correct elements.
Ray