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

TL;DR

This study uses Diffusion Monte Carlo to determine the ground state of FePc, confirming the $A_{2g}$ state and clarifying the role of exchange interactions, thus resolving ambiguities from previous models and DFT calculations.

Contribution

The paper provides the first DMC-based validation of the FePc ground state, highlighting the importance of short-range exchange and critiquing simplified ligand field models.

Findings

DMC predicts $A_{2g}$ as the ground state of FePc.

Short-range exchange interactions are crucial for stability.

Simplified models underestimate the energy of the $A_{2g}$ state.

Abstract

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.