About calculating Chi parameter

Dear all,
I'm trying to follow a tutorial for calculating cohesive energy density and later the Chi parameter. I realized that the amorphous cell was equilibrated two times using NVT and later NVE. but I don't know the reason behind this.
first, why the system was equilibrated two times and second what's the reason behind choosing these two ensembles?
I'd appreciate if anyone can provide some clarifications over this.

Thanks so much,