Benchmarking the Equation of State and B1–B2 Phase Transition of MgO Using Auxiliary-field Quantum Monte Carlo*

As a prototype rock-forming mineral in planets, a pressure calibrator in diamond-anvil cell experiments, and a window material in shock experiments, magnesium oxide (MgO) and its equation of state (EOS) and properties associated with the B1–B2 phase transition are very important to the modeling of super-Earths' interior dynamics and elucidating materials physics at extreme conditions. This problem of the B1–B2 transition in MgO has been studied for over 40 years but remains unsolved, experimentally and theoretically.

By applying auxiliary-field quantum Monte Carlo (AFQMC), we calculate the cold-curve EOS of MgO up to 1 TPa. The results reproduce experimental data at low pressures and are used to benchmark the performance of various exchange-correlation functionals in density functional theory (DFT) calculations. Our cold-curve and Hugoniot results show the PBEsol functional is in better agreement with experiments and AFQMC predictions than PBE or LDA. We then perform extensive phonon and quantum molecular-dynamics calculations with the optimal functional of PBEsol to obtain the thermodynamic free energies and quantify, with uncertainties, the effect of anharmonic vibration and electronic correlation on the B1–B2 phase boundary.

This work represents the first application of the AFQMC approach to benchmark the cold curve and phase transition in solid-state materials under very high pressure and provides a preliminary reference for the EOS and B1–B2 phase boundary of MgO from zero to 10,500 K.