Quantitative nonlinear time-dependent density functional theory (TDDFT) for large systems

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Abstract Daniel Neuhauser at the University of California, Los Angeles, is supported by an award from the Chemical Theory, Models and Computational Methods (CTMC) Program in the Chemistry Division to explore electron motion in nanomaterials in response to intense laser irradiation. This is an interesting and complex regime where new phenomena are emerging but it remains largely unexplored since accurate theoretical and computational methods for the description of excited states can only be applied to small systems. Dr. Neuhauser and coworkers are developing new methods which incorporate the necessary quantum mechanical effects at moderate computational costs. A novel computational framework is being developed that merges quantum chemistry, condensed matter physics and applied mathematics through a probabilistic approach to the problem. Specific efforts are being made to involve undergraduate and high school students, and the project emphasizes leadership opportunities for graduate students and postdocs. To obtain the fundamental physical characteristics of complex systems, a fully ab-initio quantum description is necessary. The most suitable approach is to employ real time Time-Dependent Density Functional Theory (TDDFT) with long-range exact exchange which approximates quasiparticle energies and includes attractive electron-hole interactions. The latter are necessary to account for excitonic effects and the redistribution of spectral weight in absorption cross sections. However, conventional implementations severely limit the maximum size of the system that can be treated using TDDFT with exact exchange. The project develops a stochastic TDDFT method which simulates the exchange interaction stochastically, i.e., summations over all occupied states are replaced by random sampling of the occupied Hilbert subspace, while the remaining parts of the description can be simulated traditionally. This makes it possible to treat systems with thousands of atoms and calculate the nonlinear response for systems that could not be handled previously. Description of the optical response of highly inhomogeneous systems, e.g., defect states, is being treated with a new Embedded Stochastic TDDFT, which combines stochastic and deterministic TDDFT for different regions of space. Mentoring of students ranging from high school to postdoctoral status is provided through this work, and the software developed is being made available to the wider community. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.