Ionic crystal behaviour in Gromacs

GROMACS version: gromacs/2021.5-gcc11.2.0-mkl
GROMACS modification: No
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I am dealing with molecular dynamic simulations of ions in vacuum using Gromacs .I have two crystals that is each of Sodium Bromide and Potassium Chloride.
I want to simulate these crystals in vaccuum but at different temperatures that is NaBr crystal at 3000K and KCl crystal at 300K in the same box and study their collisions in the system.
What changes shall I do in tc-grps parameters in the mdp file or any other parameter so that I can acheive this?