Quantum Monte Carlo calculations on MgO

Scientific problem

Highly correlated systems, such as minerals containg d- and f-electrons, as well as crystals described by van der Waals interactions cannot be described with standard DFT calculations. One approach that has recently been employed to study highly correlated systems is the quantum Monte-Carlo (QMC) technique. The QMC method is computational very demanding, but it is getting more tractable with the increase in computer power, and because the algorithms are essentially parallel we have the possibility to study systems with thousands and more electrons.

This is one of the first ever attempts to study oxides using the QMC technique. All calculations have been run on the HPCx as they are highly parallel and about 1000 times more expensive than traditional DFT calculations. The required wavefunction files are sevaral gigabytes, and we use the SRB to store these files

Scientific results

Here we summarise the major achievements concerning our QMC calculations on MgO.

• Optimised structural and elastic properties of the B1 structure are in excellent agreement with experiment when corrected for zero-point motions and pseudo-potential error: the calculated lattice parameter is 4.23 Å, and the bulk modulus 158 GPa. The corresponding experimental values are 4.21 Å and 160±2 GPa.
• The predicted value of the pressure transition, 597±20GPa, is in good agreement with the LDA value (569 GPa) using the same pseudo-potentials (PP) as in the QMC calculations. No experimental value is available. This has been achieved because the total energies were determined with a statistical error bar of less than 10 meV/atom.

Credits

This work was carried out by M. Alfredsson, J. Brodholt, D. Price, D. Alfe, M. Gillan (UCL) and M. Towler and R. Needs (Cambridge).