Description

The objective of this PR is to enable external potentials to be used for both SAPT(DFT) and SAPT(HF).

User API & Changelog headlines

• Enables users to call psi4.energy('sapt(dft)', external_potential=<EXT_POTENTIAL>) and acquire SAPT(HF) and SAPT(DFT) energies in the presence of an external potential.
• User can also write external potentials as any combination of C, A, and/or B with the dictionary external potential format.

Dev notes & details

• The present implementation leverages the scf_helper() external_potential logic to generate and re-use the jk-object to obtain the correct SAPT external potential energies; however, this does not re-use the logic of FISAPT's external potential code. Another ongoing implementation of external potentials in SAPT(DFT) can be found on this branch for long term goals of migrating FI-SAPT functionality into SAPT(DFT).
• This PR depends on PR#3256 for re-using jk-objects and PR#3232 for automatic GRAC shifts. It should be noted that neither of these PR's seem necessary for the proposed changes to work, but rather, this branch started with both of those changes.

Questions

• Do we require any additional information about the external potential energies for SAPT(HF) and SAPT(DFT) that isn't provided from the scf_helper() route of re-using jk-objects? Most of the logic in FISAPT0 that is neglected is for handling FI-SAPT. While FI-SAPT exernal potential setup calls are abstracted to be used for SAPT(DFT) if desired in the future, SAPT(DFT) implements electrostatic embedding through the ExternalPotential class on the python side.

Checklist

• Tests added for any new features (see ./tests/pytests/test_saptdft.py with specifically test_sapthf_external_potential() corresponding to the FISAPT0 test located psi4/tests/fsapt-ext/input.dat.
• All or relevant fraction of full tests run

Status

• Ready for review
• Ready for merge