How electrons move and respond to fields is the key to understanding properties of materials at their most fundamental level. Solving the fundamental equations of quantum theory ab initio (without recourse to parameters or models) in a universal manner is extremely difficult, because electrons interact. By far the most popular methods are those based in density functional theory (DFT). They are used in almost every branch of science and engineering because they can successfully describe many kinds of materials and their properties. But DFT suffers from many well known limitations, and to overcome them a veritable zoo of quasi-ab initio extensions has evolved. But because DFT is in practice based on an ansatz, ambiguities are inevitable, making systematic improvements problematic.

An alternative approach is through Green’s function methods where G, instead of the density n, is the basic variable. G contains more information and the approximations are easier to identify, albeit at greater cost. A very successful approach has been to construct G through perturbation theory, in powers of the screened coulomb interaction W. The simplest of these is the GW approximation. Ambiguity is present because there is freedom to choose the reference Green’s function G0 which makes G and W. G0 can be chosen self-consistently, by minimizing the difference |G-G0|. At the GW level, this is called quasiparticle self-consistent GW, or QSGW.

QSGW turns out to be a rather good universal framework for describing excitations of weakly and moderately correlated systems. A few instances will be shown that highlight both its successes and limitations. QSGW works less well when correlations are strong, particularly when spin fluctuations are important. Dynamical Mean Field theory (DMFT) has long been a method designed to address strong, local correlations. We show how QSGW can be combined with DMFT, and present arguments that this combination includes most of the important elements missing from QSGW. Fe and Ni are used as illustrations.

Biography: Professor Mark van Schilfgaarde arrived in the United Kingdom in late 2011 to lead the Theory & Simulation of Condensed Matter Group at King’s College London, leaving his position as Paul Galvin Professor at Arizona State University (ASU). Before ASU, he was a Researcher at Sandia National Labs. He completed his PhD at Stanford University in 1987. He is a Fellow of the American Physical Society and sits on the executive boards of the Simons Foundation and the Thomas Young Centre. Professor van Schilfgaarde has pioneered a number of major developments in electronic structure theory and their practical implementation. Dr. van Schilfgaarde is a co-author on approximately 200 papers. Since arriving in the UK he has been invited to speak at around 40 international workshops and conferences worldwide.