Optical properties of defects in solids via quantum embedding with good active space orbitals

TL;DR

This paper evaluates active-space orbital selection schemes within quantum embedding methods to accurately predict defect excitation energies in solids, achieving high accuracy with significantly reduced computational cost.

Contribution

It introduces and compares three active-space orbital selection schemes for quantum embedding in defect studies, demonstrating their effectiveness in predicting excitation energies.

Findings

Best schemes predict excitation energies within 0.1-0.2 eV of converged values.

Achieves many-orders-of-magnitude computational savings.

CCSD predicts excitation energies with 0.1-0.3 eV accuracy.

Abstract

The study of isolated defects in solids is a natural target for classical or quantum embedding methods that treat the defect at a high level of theory and the rest of the solid at a lower level of theory. Here, in the context of active-space-based quantum embeddings, we study the performance of three active-space orbital selection schemes based on canonical (energy-ordered) orbitals, local orbitals defined in the spirit of density matrix embedding theory, and approximate natural transition orbitals. Using equation-of-motion coupled-cluster theory with single and double excitations (CCSD), we apply these active space selection schemes to the calculation of the vertical singlet excitation energy of a substitutional carbon dimer defect in hexagonal boron nitride, an oxygen vacancy in magnesium oxide, and a carbon vacancy in diamond. Especially when used in combination with a simple composite correction, we find that the best performing schemes can predict the excitation energy to about 0.1-0.2 eV of its converged value using only a few hundred orbitals, even when the full supercell has thousands of orbitals, which amounts to many-orders-of-magnitude computational savings when using correlated electronic structure theories. When compared to assigned experimental spectra and accounting for vibrational corrections, we find that CCSD predicts excitation energies that are accurate to about 0.1-0.3 eV, which is comparable to its performance in molecules and bulk solids.