A necessary step for targeted materials design is to develop accurate and affordable methods that can characterize the electronic structure of solids. Quantum embedding provides an attractive balance in the compromise between cost and accuracy - especially when studying phenomena that are localized in the real space. Here, I will talk about our recent developments using the density matrix embedding theory (DMET) algorithm to study electronic excitations in solid-state defects. For open-shell systems, we observe the better performance of a restricted open-shell Hartree--Fock (ROHF) bath when compared to the previously implemented restricted Hartree--Fock (RHF) bath. We also implement multireference impurity solvers, specifically, complete active space self-consistent field (CASSCF) and n-electron valence state second-order perturbation theory (NEVPT2). We apply the method to investigate an oxygen vacancy (OV) on a MgO(100) surface and find excellent agreement between DMET using the CASSCF/NEVPT2 solver and the non-embedded CASSCF/NEVPT2 approach. We further show that DMET can be used to find the excitation energies of much larger supercells for the OV defect and a neutral silicon-vacancy in diamond where the use of non-embedded CASSCF/NEVPT2 becomes intractable.

Local excitations in crystalline point defects with multireference density matrix embedding theory

ACS Spring 2022

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