Relative Energies and Electronic Structure of CoO Polymorphs Through ab-Initio Diffusion Quantum Monte Carlo Method*

Density Functional Theory (DFT) is currently the most popular and versatile method for atomic scale modeling and rational design of new materials. However, transition metal oxides are particularly problematic for DFT, due to the strong many-body interactions in the d-orbitals. Recently developed DFT functionals, such as SCAN, improve over the performance of LDA and PBE, but we find that they still fail to identify rocksalt as the most stable polymorph of CoO. Although DFT calculations can be adjusted by empirical parameters, such as Hubbard-U, the transferability of these parameters is questionable. Diffusion Monte Carlo (DMC) is a method that treats electrons explicitly, solving the many-body Schrödinger equation with minimum approximations. In this presentation, we will discuss our recent application of DMC methods on CoO polymorphs. We report equation of state and quasiparticle gaps data, showing that DMC is able to predict correct energetic ordering between the polymorphs. We discuss the relative importance of the errors in DMC and compare different methods in the literature used to evaluate pseudopotentials.