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This study extends quantum chemistry methods by enabling density matrix renormalization group (DMRG) with complete active space second-order perturbation theory (CASPT2) using localized orbitals. This improves calculations for complex molecular systems.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Previous work combined Density Matrix Renormalization Group (DMRG) with Complete Active Space Second-Order Perturbation Theory (CASPT2) using pseudo-canonical molecular orbitals (PCMOs).
  • The PCMO basis simplified the construction of the four-particle reduced density matrix (4-RDM) within the DMRG-CASPT2 framework.

Purpose of the Study:

  • To develop an approach for using more suitable orbital bases, such as localized molecular orbitals, in DMRG-CASPT2 calculations.
  • To extend the applicability of the DMRG-CASPT2 method to a wider range of chemical systems.

Main Methods:

  • Developed an approximation for the multiplication of the 4-RDM and generalized Fock matrix in general orbitals using cumulant reconstruction.
  • Presented an algorithm for computing the three-particle reduced density matrix (3-RDM) for DMRG wavefunctions, extending existing 2-RDM algorithms.

Main Results:

  • Successfully implemented DMRG-CASPT2 with localized molecular orbitals, overcoming the simplification offered by PCMOs.
  • Demonstrated the performance of the extended approach on large-scale multireference systems, including excited states of polyenes and copper-dioxygen complexes.

Conclusions:

  • The developed method extends the applicability of DMRG-CASPT2 by allowing the use of localized orbitals.
  • The approach provides accurate computational results for complex electronic structure problems in chemistry.