I am using GTPack v1.3 on Mathematica 12.1, and obtained an error when I tried to run the CrystalFieldSplitting routine going from Oh to D3d. I get an error that D3d is not a subgroup of Oh, when in fact it truly is a subgroup (see here).
This is a very basic operation, so am I missing something here? Or is this an error in GTP?
You are correct, D3d is a subgroup of Oh. However, keep in mind that a specific representation of the group depends on the chosen coordinate system. You install Oh and D3d using GTInstallGroup. This command installs the groups taking the z-axis as the principle axis. In that form Oh, does not have a three-fold rotation about the z-axis, but D3d. You would need to generate D3d for a threefold rotation axis in correspondence to Oh, e.g., the alpha-axis. The module you need is GTGroupFromGenerators. See an example below: