The latest SLIM is working in cesm2_3 tags, and for example this test on Derecho:
ERS_D_Ld5_PS_Vmct.f10_f10_mg37.I2000SlimRsGs.derecho_gnu.slim-default
Passes in cesm2_3_alpha17a. So it is working on Derecho, but we should make a tag that
updates it to cesm2_3_beta17 (when it's made) with standalone testing moved from
Cheyenne to Derecho. Then SLIM will be able to work on Derecho in a standalone
checkout.
The way around that would be to update the externals in the Externals.cfg file to the
externals in the latest cesm2_3_alpha17X tag.