Comparing ab-initio diffusion Monte Carlo and quantum embedding excited state calculations for an Fe³⁺ point defect in AlN

Quantitative predictions of point defect excited states aid in materials design for optoelectronics and quantum information. However, certain defect excited states – such as those of substitutional Fe_Al³⁺-in-AlN – require multi-determinant wave functions [1]. Ab-initio quantum embedding (QE) calculations balance accuracy and scalability by treating a small active space with an interacting theory. However, prior QE results for Fe³⁺-in-AlN showed qualitative dependence on double counting and functional approximations [2], so higher-accuracy reference calculations are needed.



Diffusion Monte Carlo (DMC) calculations have achieved high accuracy even for large, periodic systems using correlated, compact trial wave functions [3]. DMC excited state calculations are now possible by penalizing wave function overlaps during optimization [4]. We establish DMC’s accuracy and QE’s errors on the low-lying excited states of Fe³⁺-in-AlN by comparing their spectra against each other and experiment.



[1] M. Bockstedte, et. al., npj. Quantum Mater. 3 31 (2018).

[2] L. Muechler, et. al., Phys. Rev. B 105 235104 (2022).

[3] W. M. C. Foulkes, et. al., Rev. Mod. Phys. 73, 33 (2001).

[4] S. Pathak, et. al., J. Chem. Phys. 154 034101 (2021).