FCIQMC-Tailored Distinguishable Cluster Approach: Open-Shell Systems

A recently proposed
tailored approach based on the distinguishable
cluster method and the stochastic FCI solver, FCIQMC [J. Chem.
Theory Comput. 2020, 16, 5621], is extended to open-shell
molecular systems. The method is employed to calculate spin gaps of
various Fe­(II) complexes, including a Fe­(II) porphyrin model system.
Both distinguishable cluster and fully relaxed CASSCF natural orbitals
were used in this work as reference for the subsequent tailored distinguishable
cluster calculations. The distinguishable cluster natural orbitals
occupation numbers were also used as an aid to the selection of the
active space. The effect of the active space sizes and of the explicit
correlation correction (F12) onto the predicted spin gaps is investigated.
The tailored distinguishable cluster with singles and doubles yields
consistently more accurate results compared to the tailored coupled
cluster with singles and doubles.