Development of a variational APW method with its application to the electronic structure of superionic conductors

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We have made details of the variational Augmented Plane Wave (APW) method available to the public. The new method determines the Kohn-Sham (KS) potentials in the framework of the muffin-tin (MT) approximation through minimization of the total energy functional of a system, including the radii of the MT spheres. It was confirmed that the fundamental idea of the method was applicable to other electronic-structure calculation schemes such as the LMTO and ASW methods with adjustable parameters for the KS potentials. The variaional APW method was then applied to a series of noble-metal halides to demonstrate that combination of multipole polarizability of the d shell and weakness of the p-d hybridization plays a primary role in giving rise to the superionic conductivity. We introduced a new parameter, called generalized covalency, which quantifies the strength of hybridization between the p bands arising from the p states of host halogen atoms and the d bands from the d states of noble-metal atoms. The generalized covalency systematically explains the trend of ionic conductivity as shown by the compounds. We have also carried out the tight-binding (TB) calculations for the generalized covalency in the noble-metal compounds. Appropriate choice of the TB parameters leads to the essentially same results as those obtained by the first-principle calculations. Currently we are developing a molecular dynamics code based on the TB approximation that enables us to study the ionic transport in materials at the electronic level ; parallel computation based on the Message Passing Interface is within our scope. How to extract the TB parameters from the variational APW calculations is a remaining issue to be addressed.