FCIQMC-CASPT2 with Imaginary-Time-Averaged Wave Functions

A new method to perform complete active space second-order
perturbation
theory on top of large active spaces optimized with full configuration
quantum Monte Carlo is presented. Computing the three- and Fock-contracted
four-particle density matrix from imaginary-time-averaged wave functions
is found to resolve fermionic positivity violations and to ensure
numerical stability. The protocol is applied to [NiFe]-hydrogenase,
[Cu2O2]-oxidase and Fe-porphyrin model systems
up to 26 electrons in 27 orbitals and benchmarked against DMRG-CASPT2.