Hi @JoshLipman,
Just summarising what @sjahangiri nicely explained, assuming you are using conda.
You can create a new conda environment with:
conda create --name name_of_your_environment python=3.9
Activate the environment:
conda activate name_of_your_environment
and after this you can install the needed packages:
pip install numpy pennylane pennylane-qchem
Once you have performed these steps install psi4:
conda config --add channels http://conda.anaconda.org/psi4
conda install psi4
Installing psi4 can take some time, in some cases.
Once you have followed these steps you should be OK to run the example provided by @sjahangiri.
Let us know if this worked for you.