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 optimised with full configuration quantum Monte Carlo is presented. Three– and four–particle density matrices, including contractions with the Fock matrix, are computed from imaginary–time–averaged wave functions, which resolves fermionic positivity violations and ensures numerical stability. The protocol is applied to [NiFe]$^{2-}$ , [Cu(NH$_3$)]$_2$O$_2^{2+}$ and Fe–porphyrin model systems up to 26 electrons in 27 orbitals and benchmarked against DMRG–CASPT2.