Investigation of theoretical basis of DFT accuracy and study of ferroelectric disordered solution by a multiscale approach

First-principles pseudopotential density functional theory (DFT) calculations have become a vital tool in research on a variety of physical, chemical and material science problems. Limiting the effect of the uncontrolled approximations is crucial for improvement of accuracy and reliability of first-principles methods. Study of disordered ferroelectric solutions has benefited from first-principles investigations but still remains a challenging scientific problem. In this thesis, we present our research into estimating and eliminating errors in previously uncontrolled pseudopotential and exchage-correlation approximations as well as our studies of the PZT solid-state solution used in current piezoelectric devices. We present an improved method for construction of relativistic pseudopotentials as well as two studies of new criteria for pseudopotential transferability. As the quality of the exchange-correlation functional approximations used in DFT calculations improves, the error introduced by inexact pseudopotential construction will become more significant. Therefore, our work is an important step in the direction of fully accurate calculations. We investigate the reasons for incorrect site preferences found by theoretical calculations for a variety of molecule-surface systems, using CO/Pt(III) as a prototypical adsorption system. We determine that the inaccuracy in the DFT CO bond breaking energy leads to the discrepancy between experimental and theoretical results. We also study the local structure of PZT, using DFT calculations to characterize which atomic properties are transferable, independent of local environment and which are non-transferable and environment-dependent. This is important for understanding the accuracy of the crystal chemistry approach to solid solutions. In the last project, we use a multiscale approach combining DFT and phenomenenological model calculations to show how changes in the statistical population of local Pb environments with Zr/Ti composition coupled with disorder result in compositional phase transitions and favorable properties of PZT