Quantum Embedding Methods to investigate oxygen vacancies in Bulk MgO

Quantum embedding methods enable treating different parts of a system at distinct levels of electronic structure theory. For instance, the strongly correlated states of defects in solids may be treated at a higher level of theory than the rest of the system. Quantum defect embedding theory (QDET) [1,2] and periodic density matrix embedding theory (pDMET) [3] are two different methods, the former based on Green’s function embedding, and the latter using the Schmidt decomposition of the system’s density matrix. In this work we employ QDET (and the WEST code) and pDMET (and the PySCF code) to investigate the F⁰ center (neutral oxygen vacancy) in bulk MgO, which gives rise to a singly occupied localized defect state within the solid band gap. We investigate the orbital character of the defect state and compare the extrapolated vertical excitation energies obtained from the two embedding techniques. The active spaces derived from QDET and DMET are in good agreement with each other, however some of the computed excitation energies between multi-reference states differ. Work is in progress to analyze in detail the impact of all the approximations involved in QDET and DMET on the computed vertical excitation energies and compare them with experiments.