Spin-pure Stochastic-CASSCF via GUGA-FCIQMC applied to Iron Sulfur Clusters

Abstract

In this work we demonstrate how to compute the one- and two-body reduced density matrices within the spin-adapted 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 whilst 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 procedure based on restricted open-shell Hartree-Fock orbitals. We show that 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 CI eigenvalues are well fit by a simple bilinear Heisenberg Hamiltonian, the CASSCF eigenvalues exhibits deviations that necessitate the inclusion of biquadratic terms in the model Hamiltonian.

arXiv comment19 pages, 17 Figures

References in corpus (29)

• The density-matrix renormalization group in the age of matrix product states
• The density-matrix renormalization group
• Heat-bath Configuration Interaction: An efficient selected CI algorithm inspired by heat-bath sampling
• Semistochastic Heat-bath Configuration Interaction method: selected configuration interaction with semistochastic perturbation theory
• Orbital Optimization in the Density Matrix Renormalization Group, with applications to polyenes and β-carotene
• A deterministic alternative to the full configuration interaction quantum Monte Carlo method
• Low-energy spectrum of iron-sulfur clusters directly from many-particle quantum mechanics
• A spin-adapted Density Matrix Renormalization Group algorithm for quantum chemistry
• Cheap and near exact CASSCF with large active spaces
• Modern Approaches to Exact Diagonalization and Selected Configuration Interaction with the Adaptive Sampling CI Method
• Semistochastic Projector Monte Carlo Method
• An Efficient Matrix Product Operator Representation of the Quantum-Chemical Hamiltonian
• Pushing Configuration-Interaction to the Limit: Towards Massively Parallel MCSCF Calculations
• CASSCF with Extremely Large Active Spaces using the Adaptive Sampling Configuration Interaction Method
• Hybrid stochastic-deterministic calculation of the second-order perturbative contribution of multireference perturbation theory
• A general second order complete active space self-consistent-field solver for large-scale systems
• Understanding the mechanism stabilizing intermediate spin states in Fe(II)-Porphyrin
• Unbiased Reduced Density Matrices and Electronic Properties from Full Configuration Interaction Quantum Monte Carlo
• Selected configuration interaction dressed by perturbation
• Efficient heat-bath sampling in Fock space
• Linear-scaling and parallelizable algorithms for stochastic quantum chemistry
• An excited-state approach within full configuration interaction quantum Monte Carlo
• Semi-stochastic full configuration interaction quantum Monte Carlo: developments and application
• NECI: N-Electron Configuration Interaction with emphasis on state-of-the-art stochastic methods
• Efficient Formulation of Full Configuration Interaction Quantum Monte Carlo in a Spin Eigenbasis via the Graphical Unitary Group Approach
• Second-Order Self-Consistent-Field Density-Matrix Renormalization Group
• Density matrices in full configuration interaction quantum Monte Carlo: Excited states, transition dipole moments and parallel distribution
• Microscopic origin of isotropic non-Heisenberg behavior in highly correlated systems
• Exciting determinants in Quantum Monte Carlo: Loading the dice with fast, low memory weights