New Insight into the Ground State of FePc: A Diffusion Monte Carlo Study

We have applied DMC to evaluate relative stability of the possible electronic configurations of an isolated FePc under $D_{4h}$ symmetry, considering some fixed nodes generated from different methods. They predict $A_{2g}$ ground state consistently, supporting preceding DFT studies, with confidence overcoming the ambiguity about exchange-correlation (XC) functionals. By comparing DMC with several XC, we clarified the importance of the short range exchange to describe the relative stability. We examined why the predicted $A_{2g}$ is excluded from possible ground states in the recent ligand field based model. Simplified assumptions made in the superposition model are identified to give unreasonably less energy gain for $A_{2g}$ when compared with the reality. The state is found to have possible reasons for the stabilization, reducing the occupations from an unstable anti-bonding orbital, avoiding double occupation of a spatially localized orbital, and gaining exchange energy by putting a triplet spin pair in degenerate orbitals.