All the DMET calculations were performed using our in-house pDMET and mrh codes which utilizes the electron integrals and quantum chemical solvers from PySCF. Wannierization was done using the wannier90 53 code via the pyWannier90 interface. The Goedecker-Teter-Hutter pseudopotentials were used for all the calculations. The geometry optimizations were performed at the spin-unrestricted PBEsol level 57 using the Vienna Ab initio Simulation Package (VASP). The convergence criteria of 10 -6 eV and 10 -3 eV/ Å were used for the energy and force, respectively. We represent a MgO(100) surface using a single layer of Mg and O with the chemical formula Mg 18 O 18 . We performed benchmark calculations on two point defects, namely the oxygen mono-vacancy (OV) and a oxygen di-vacancy (OOV). For these systems, we computed singlet-singlet and singlettriplet excitation energies using CAS-DMET, NEVPT2-DMET and pDME-PDFT. We used the translated PBE functional for both PDFT and hybrid PDFT which are referred to as pDME-tPBE and pDME-tPBE0 respectively. The oxygen mono-vacancy defect is created by removing one neutral oxygen atom at the center of the unit cell. The di-vacancy is created by removing an additional neutral oxygen atom nearest to the mono-vacant oxygen atom.