Hi, GPUMD is great to use for studying thermal transport using ML potentials, but I was wondering if the flexibility of two commands in the code be improved. The fix command only allows 1 group to be fixed per simulation run, i.e

fix 1
fix 9 

will result in only atoms in group 9 of grouping method 0 to be fixed when the run command is called. This can be inconvenient for NEMD simulations where the ends of the structure are fixed to prevent translation. The issue is bypassed be adding the atoms in group 1 to group 9 in the model.xyz file, but allowing the command to fix multiple groups of atoms like

fix 1 9

would be more convenient, in my opinion. Similarly I was wondering if the same could be done for compute_shc. I understand that there is an option for allowing the SHC to be computed for ALL groups in the grouping method when

compute_shc 1 500 1 500 200.0 group 1 -1

is used. but could the command also allow

compute_shc 1 500 1 500 200.0 group 1 1 2 3 ...

for however many groups the user would like to find SHC of.

Thanks again for all the developers' work on GPUMD, it is fantastic!

Mayur