Quantum Monte Carlo calculations of the structural properties and the B1-B2 phase transition of MgO

We report diffusion Monte Carlo (DMC) calculations on MgO in the rock-salt and CsCl structures. The calculations are based on Hartree-Fock pseudopotentials, with the single-particle orbitals entering the correlated wave function being represented by a systematically convergeable cubic-spline basis. Systematic tests are presented on system-size errors using periodically repeating cells of up to over 600 atoms. The equilibrium lattice parameter of the rocksalt structure obtained within DMC is almost identical to the Hartree-Fock result, which is close to the experimental value. The DMC result for the bulk modulus is also in good agreement with the experimental value. The B1-B2 transition pressure (between the rocksalt and CsCl structures) is predicted to be just below $600\mathrm{GPa}$, which is beyond the experimentally accessible range, in accord with other predictions based on Hartree-Fock and density functional theories.

Figure 1

The standard error in the energy $\sigma$ for runs of the same length, calculated within VMC without a Jastrow factor, as a function of the plane-wave cut-off energy, for a 16-atom cell of MgO in the rocksalt structure with a lattice parameter of $4.17\mathrm{\AA }$.

Figure 2

The DMC energy per atom for MgO in the rocksalt structure with a volume per atom of $9.06{\mathrm{\AA }}^3$ as a function of the number of atoms in the repeating cell, using both the Ewald interaction (circle) and the MPC interaction (squares).

Figure 3

The VMC energy per atom for MgO in the CsCl structure with a volume per atom of $8.77{\mathrm{\AA }}^3$ as a function of the number of atoms in the repeating cell, calculated using the Ewald interaction. Squares and circles: simple cubic cell with $\Gamma$-point and zone-boundary sampling, respectively; diamonds and triangles: fcc cell with $\Gamma$-point and zone-boundary sampling (see text for wave vector used in zone-boundary sampling). The lines are guides to the eye.

Figure 4

The DMC energy per atom as a function of volume for MgO in the NaCl structure.

Figure 5

The DMC energy per atom for the NaCl structure (circles) and the CsCl structure (squares) of MgO as a function of volume in the region of the B1-B2 phase transition.

Figure 6

The enthalpy per atom of the NaCl and CsCl (dashed line) structures of MgO as a function of pressure. Top picture: fit to the DMC data; bottom picture: fit to the DFT-LDA data.