Estimate the ground-state energy of a molecular active space by sample-based quantum diagonalization (SQD). Hartree–Fock orbitals define the space; a CCSD-seeded LUCJ circuit is sampled on the Qiskit Aer simulator, and the sampled configurations are recovered and diagonalized. Each run also reports the exact CASCI energy of the same space, so the SQD error is visible.
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Examples
Molecule
One 3D structure with explicit hydrogens, at most 20 atoms. Mark radicals with an M RAD line so no hydrogens are implied.
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Reference and active space
Choose the basis, charge, and spin, then the active space SQD diagonalizes: at most 8 spatial orbitals.
Sampling and diagonalization
Configurations are sampled from a circuit, recovered, and diagonalized in batches.
Backend: Qiskit Aer simulator, a classical simulation of the circuit. No quantum hardware is used.