MSD is calculated from the water molecules generated by the reaxff

Hello everyone:
The force field I use is thereaxff, and my modeling idea is the water generated by the oligomer reaction, and then the water molecules will reach a constant value after relaxation and equilibrium, and then I want to do MSD on the generated water molecules.
Also, I found the same analysis in similar papers: this is the URL of the paper( [Carbon nanotube-geopolymer nanocomposites: A molecular dynamics study of the influence of interfacial chemical bonding upon the structural and mechanical properties - ScienceDirect])
I wonder how to define water group?