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

Tom Ichibha,Kenta Hongo,Ryo Maezono,Zhufeng Hou

doi:10.1038/s41598-017-01668-6

Abstract

We have applied DMC to evaluate relative stability of the possible electronic configurations of an isolated FePc under D4h symmetry, considering some fixed nodes generated from different methods. They predict A2g 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 A2g 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 A2g 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.

Highlights

Iron(II) Phthalocyanine (FePc) attracts recent interests for its potentials in spintronics[1,2,3] because it possesses the strong magnetic anisotropy as a molecular magnet[4]
Predictions of the relative stability among the states are shown in Fig. 1, compared with each other among diffusion Monte Carlo (DMC), CASSCF, and DFT
All the DMC simulations predict the ground state to be A2g, supporting several preceding DFT results[6, 14, 15], though they have been regarded not well convincing because of the ambiguity about XC, as some DFT results predict different configuration[13, 15, 26]

Summary

Electronic configuration

Coupling is taken into account, it predicts a hybrid state between Eg(b) and B2g as the ground state, latter of which is predicted as the first excited state when without the coupling. The original ligand field model for D4h requires three ligand parameters to identify the possible ground state, where A2g still remains as a possibility[16], not conflicting with ab initio DFT predictions. In a recent work[17], the possible ground state is specified by reduced two parameters and A2g has disappeared from the possibility, leading to an apparent contradiction to DFT predictions. The present study targets to investigate the apparent discrepancy about A2g ground state between ab initio and ligand field model[17] approaches. While there are two preceding studies reporting the possible geometry, the one from X-ray diffraction of β phase[24] and the other from DFT geometry optimization applied to an isolated complex[25], we used the latter for the present calculation (See Supplementary Information). Any occupations to dx2−y2 are excluded from the possibility because the orbital makes a strong σ*-coupling with neighboring ligands to get unstabilized

Results

Conclusion

Additional Information