Volodymyr Turkowski (University of Central Florida)
Development and application of DFT+DMFT and TDDFT+DMFT techniques for nanosystems
We have developed computational approaches to examine ground state properties, excitations and nonequilibrium response of strongly correlated nanostructures. The algorithm is based on merging Dynamical Mean-Field Theory (DMFT) with both Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT). The DMFT approach has already been established as a reliable tool to study correlation effects in terms of effective model Hamiltonians (Hubbard-type). It has also been successfully combined with DFT for calculations of the static properties of strongly correlated extended systems. The main element of the success of the theory is based on its ability to describe effects of dynamical fluctuations, missed in DFT+U and other approaches. These effects are, however, crucial for cases in which the local Coulomb repulsion energy U and the kinetic energy of the electrons are comparable, as found most often in correlated materials. Our recent extension of DFT+DMFT to nanosystems  demonstrates the feasibility of the method and shows that magnetic properties of 10-100-atom transition-metal structures can be obtained with computational cost comparable to that of standard DFT calculations. We have further generalized the approach to time-dependent cases by deriving the DMFT expression for the key element of the TDDFT theory – exchange-correlation kernel , and applied it to several bulk and nanoscale systems in which we expect electron correlations to play an important role: YTiO3, Ce, Ce2O3 and VO2. We demonstrate that contrary to standard TDDFT, our approach is capable of describing correctly the excitation spectrum as well as nontrivial nonequilibrium response of correlated systems. Technical simplicity and physical transparency of our proposed methods allow their extension to systems containing few hundred atoms in complex environment. The details of the techniques and their application to nanoscale systems will be discussed in the context of available experimental and other theoretical results.
1. V. Turkowski, A. Kabir, N. Nayyar, and T.S. Rahman, J. Phys.: Cond. Mat. (Fast Track) 22, 462202 (2010); J. Chem. Phys. 136, 114108 (2012).
2. V. Turkowski, T.S. Rahman, J. Phys.: Cond. Mat. (Fast Track) 26, 022201 (2014).