Check on a simple case whether the cooling works correctly without any radiation being present: Use a uniform 3D box of hot gas and let it cool down. The reference solution assumes case A recombination. To run with GEAR-RT, compile swift with: --with-rt=GEAR_3 --with-rt-riemann-solver=GLF --with-hydro=gizmo-mfv --with-riemann-solver=hllc --with-stars=GEAR --with-feedback=none --with-grackle=$GRACKLE_ROOT