Fragmentation methods applied to multireference wave functions constitute a road towards the application of highly accurate ab initio wave function calculations to large molecules and solids. However, it is important for reproducibility and transferability that a fragmentation scheme be well-defined with minimal dependence on initial orbital guesses or user-designed ad hoc fragmentation schemes. One way to improve this sort of robustness is to ensure the energy obeys a variational principle; i.e., that the active orbitals and active space wave functions minimize the electronic energy in a certain ansatz for the molecular wave function. We extended the theory of the localized active space self-consistent field, LASSCF, method (JCTC 2019, 15, 972) to fully minimize the energy with respect to all orbital rotations, rendering it truly variational. The new method, called vLASSCF, substantially improves the robustness and reproducibility of the LAS wave function compared to LASSCF. We analyze the storage and operation cost scaling of vLASSCF compared to orbital optimization using a standard CASSCF approach and we show results of vLASSCF calculations on some simple test systems. We show that vLASSCF is energetically equivalent to CASSCF in the limit of one active subspace, and that vLASSCF significantly improves upon the reliability of LASSCF energy differences, allowing for more meaningful and subtle analysis of potential energy curves of dissociating molecules. We also show that all forms of LASSCF have a lower operation cost scaling than the orbital-optimization part of CASSCF.
Exactly-embedded multiconfigurational self-consistent field theory using density matrix embedding: the localized active space self-consistent field method
Measurement Reduction in Orbital-Optimized Variational Quantum Eigensolver via Orbital Compression
State Key Laboratory of Precision and Intelligent Chemistry, University of Science and Technology of China（精密与智能化学国家重点实验室，中国科学技术大学）；Hefei National Laboratory, University of Science and Technology of China（合肥国家实验室，中国科学技术大学）
Sample-based quantum diagonalization as parallel fragment solver for the localized active space self-consistent field method
Pritzker School of Molecular Engineering, University of Chicago（芝加哥大学普里茨克分子工程学院）；IBM Quantum, IBM T.J. Watson Research Center（IBM量子，IBM T.J.沃森研究中心）；Department of Chemistry, Chicago Center for Theoretical Chemistry, University of Chicago（芝加哥大学化学系，芝加哥理论化学中心）；IBM Quantum, IBM Research -- Almaden（IBM量子，IBM阿尔马登研究中心）；IBM Quantum, IBM Research -- Tokyo（IBM量子，IBM东京研究中心）；JSR Corporation（JSR公司）
