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LAS-QC

Post-LAS quantum algorithms endeavor to improve upon the fragment-based LASSCF method by offloading the expensive reconstruction of the complete active space onto a quantum computer. LAS-UCC is a hybrid quantum-classical algorithm that involves loading a classically converged fragment wave function onto fragment qubits, and using the variational quantum eigensolver (VQE) algorithm with a generalized Unitary Coupled Cluster ansatz to recover correlation between fragments. LAS-USCC or LAS Unitary Selective Coupled Cluster begins from the same generalized UCC ansatz but classically pre-selects amplitudes based on a gradient expression, to provide a compact circuit for the VQE. LAS-QKSD is a hybrid quantum-classical algorithm that involves a similar state preparation step in the fragments, and then uses a Krylov subspace of time-evolved states as the basis of diagonalization to obtain the ground state energy. LAS-nuVQE involves a non-orthogonal variant of VQE, with classical Jastrow-inspired parameters that are simultaneously optimized at the same time as the quantum ansatz parameters.

LAS-QC is the in-house code for performing all post-LAS calculations. It requires:

  1. Qiskit version 1.x.x
  2. Qiskit-nature version 0.x
  3. MRH
  4. PySCF version 2.x

The code and installation instructions may be found on Github: https://github.com/GagliardiGroup/las-qc

Developer:

  1. Ruhee D’Cunha
  2. Shreya Verma
  3. Qiaohong Wang
  4. Dustin Broderick

Git Link:: https://github.com/GagliardiGroup/las-qpe

References:: 

  1. Otten, M.; Hermes, M. R.; Pandharkar, R.; Alexeev, Y.; Gray, S. K.; Gagliardi, L. Localized Quantum Chemistry on Quantum Computers. J. Chem. Theory Comput. 2022, 18, 12, 7205–7217. https://doi.org/10.1021/acs.jctc.2c00388.
  2. D’Cunha, R.; Otten, M.; Hermes, M.R.; Gagliardi, L; Gray, S. State preparation in quantum algorithms for fragment-based quantum chemistry. J. Chem. Theory Comput. 2024, 20, 8, 3121–3130. https://doi.org/10.1021/acs.jctc.3c01283
  3. Mitra, A.; D’Cunha, R.; Wang, Q.; Hermes, M.R.; Alexeev, Y.; Gray, S. K.; Otten, M.;  Gagliardi, L. The Localized Active Space Method with Unitary Selective Coupled Cluster. J. Chem. Theory Comput. 2024, 20 (18), 7865–7875. https://doi.org/10.1021/acs.jctc.4c00528
  4. D’Cunha, R.; Cortes, C. L.; Gagliardi, L.; Gray, S. K. Fragment-Based Initialization for Quantum Subspace Methods. Phys. Rev. A 2024, 110 (4), 042613. https://doi.org/10.1103/PhysRevA.110.042613
  5. Wang, Q.; D’Cunha, R.; Mitra, A.; Gray, S. K.; Otten, M.; Gagliardi, L. Non-unitary Variational Quantum Eigensolver with the Localized Active Space Method and Cost Mitigation, arXiv, 2025. DOI: https://doi.org/10.48550/arXiv.2501.13371
  6. Verma, S.; D’Cunha, R.; Mitra, A.; Hermes, M. R.; Gray, S. K.; Otten, M.; Gagliardi, L. Polynomial Scaling Localized Active Space Unitary Selective Coupled Cluster Singles and Doubles, ChemRxiv. 2025; https://doi.org/10.26434/chemrxiv-2025-1n3d4-v2.