AI Article Synopsis
- The study introduces a method to compute one- and two-body reduced density matrices efficiently using full configuration interaction quantum Monte Carlo (FCIQMC) based on the graphical unitary group approach (GUGA).
- This method enhances the capability to stochastically treat larger active spaces compared to traditional configuration interaction (CI) solvers while allowing variational relaxation of orbitals for specific spin-pure states.
- The research focuses on iron-sulfur model systems, revealing that orbital relaxation significantly affects magnetic coupling parameters, highlighting differences in results between CASSCF and CI-only approaches, and suggesting the need for more complex Hamiltonian terms in modeling.
Article Abstract
In this work, we demonstrate how to efficiently compute the one- and two-body reduced density matrices within the full configuration interaction quantum Monte Carlo (FCIQMC) method, which is based on the graphical unitary group approach (GUGA). This allows us to use GUGA-FCIQMC as a spin-pure configuration interaction (CI) eigensolver within the complete active space self-consistent field (CASSCF) procedure and hence to stochastically treat active spaces far larger than conventional CI solvers while variationally relaxing orbitals for specific spin-pure states. We apply the method to investigate the spin ladder in iron-sulfur dimer and tetramer model systems. We demonstrate the importance of the orbital relaxation by comparing the Heisenberg model magnetic coupling parameters from the CASSCF procedure to those from a CI-only (CASCI) procedure based on restricted open-shell Hartree-Fock orbitals. We show that the orbital relaxation differentially stabilizes the lower-spin states, thus enlarging the coupling parameters with respect to the values predicted by ignoring orbital relaxation effects. Moreover, we find that, while CASCI results are well fit by a simple bilinear Heisenberg Hamiltonian, the CASSCF eigenvalues exhibit deviations that necessitate the inclusion of biquadratic terms in the model Hamiltonian.
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| http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8444347 | PMC |
| http://dx.doi.org/10.1021/acs.jctc.1c00589 | DOI Listing |
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