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PyPPM is an open-source Python package built on PySCF for calculating response properties using the Polarization Propagator formalism at various levels of approximation and frameworks. It currently supports RPA and Higher-RPA calculations of non-relativistic spin-spin couplings. Future versions aim to extend this to the Second-Order Polarization Propagator Approach (SOPPA) and include relativistic calculations of J-coupling and shielding.
In the non-relativistic framework, PyPPM can perform calculations using localized molecular orbitals for both occupied and virtual sets, and analyze individual coupling pathway contributions. Additionally, this software can compute quantum entanglement between virtual excitations based on the Principal Propagator, a key element of the Polarization Propagator formalism.