Combined cluster and atomic displacement expansion for solid solutions and magnetism

Abstract

Finite temperature disordered solid solutions and magnetic materials are difficult to study directly using first principles calculations, due to the large unit cells and many independent samples that are required. In this work, we develop a combined cluster expansion and atomic displacement expansion, which we fit to first principles energies, forces, and stresses. We then use the expansion to calculate thermodynamic quantities at nearly first principles levels of accuracy. We demonstrate that by treating all the relevant degrees of freedom explicitly, we can achieve improved convergence properties as compared to a simple cluster expansion, and our model naturally includes both configurational and vibrational entropy. In addition, we can treat coupling between structural and chemical or magnetic degrees of freedom. As examples, we use our expansion to calculate properties of Si${}_{1-x}$Ge${}_x$, magnetic MnO, Al with vacancies, and Ba${}_x$Sr${}_{1-x}$TiO${}_3$.

References in corpus (6)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Restoring the density-gradient expansion for exchange in solids and surfaces
• Generalized gradient approximation for solids and their surfaces
• Lattice dynamics and electron-phonon coupling calculations using non-diagonal supercells
• Efficient systematic scheme to construct second-principles lattice-dynamical models
• The Magnetic Structure of Paramagnetic MnO