Non-Hermitian Quantum Mechanics for Chemistry and Catalysis
Abstract
This proposal seeks funding for developing a theoretical framework for studying metastable electronic states (resonances). Metastable electronic states are ubiquitous in highly energetic environments and are routinely formed in electron-molecule collisions. Resonances are key players in plasma assisted ignition and combustion. Temporary ion states also govern chemistry under intense radiation conditions, for example, in plasmonic photocatalysis. Yet, the theoretical description of their energetics, lifetimes, and dynamics remains problematic. The aim of the program is three-fold: (i) enable robust correlated treatment of Feshbach and multiply-excited resonances; (ii) incorporating nuclear motion via Born-Oppenheimer ab initio dynamics models; (iii) integration of electronic structure methods for resonances with density functional embedding approaches for the description of chemical reactions on metal surfaces. Importantly, the proposed methods will be applicable to realistic molecular systems (~ 50-100 atoms). Electronic resonances metastable with respect to electron ejection pose significant challenge for quantum chemistry methods as the states belong to the continuous spectrum of the Hamiltonian (scattering states) and cannot be described using conventional electronic structure techniques developed for bound states. Non-Hermitian quantum mechanics methods, and complex absorbing potential approach in particular, surpass this problem: a resonance appears as a single square-integrable state of the non-Hermitian Hamiltonian with complex eigenvalues. Real part and imaginary part of the eigenvalue correspond to the resonance position and width, respectively. Thus, non-Hermitian quantum mechanics allow one to extend the capabilities of highly-accurate conventional quantum-chemistry methods to description of metastable electronic states. Here, we propose new models for description of energetics, lifetimes, and dynamics of metastable electronic states. Specifically, a new electronic structure method is proposed to enable calculations of Feshbach and multiply-excited resonances positions and widths. These resonances cannot be reliably described by the existing formulations of complex-variable many-body approaches for systems beyond several atoms. The method is based on perturbative treatment of complex absorbing potential in the framework of extended multiconfigurational quasidegenerate perturbation theory, XMCQDPT2. Importantly, in contrast to all previously developed non-Hermitian methods, the proposed technique enables evaluation of resonance position and width at a cost of single electronic structure calculation. As a resonance s decay via electron ejection and nuclear relaxation often occur on the same time-scale, taking into account nuclear motion is crucial for understanding electron-molecular interactions. Born-Oppenheimer ab initio dynamics on complex potential energy surfaces will be implemented in the on-the-fly manner to describe coupled electron ejection and nuclear motion. The simplicity of the model makes it applicable to large molecular systems. Finally, a hybrid approach combining electronic structure methods for resonances with density embedding techniques is proposed to account for metastable character of temporary states involved in reactions on metal surfaces. The method will enable description of resonances serving as gateway states for plasmonic photocatalysis. The new methodologies will be implemented in Q-Chem and Firefly packages, popular quantum chemistry software used in teaching, industry, and research. The developed codes will be also distributed as open-source modules via the PI website. The program will promote our understanding of electron-molecule interactions. The computational studies will be performed in collaboration with the experimental groups of Prof. Sylwia Ptasinka (U of Notre Dame & NDRL) and of Dr. Juraj Fedor (Herovsky Institute of Physical Chemistry, Prague).
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 14, 2019
- Source ID
- W911NF1910072
Entities
People
- Ksenia B. Bravaya
Organizations
- Army Contracting Command
- Boston University
- United States Army