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First-Principles Simulation Program for Excited States

Y. Noguchi, M. Hiyama, and N. Koga

These days the first-principles simulations are commonly used as a basic tool for the material research. Most of the simulation programs have been designed for the ground state only, while excited states are the target of increasing number of spectroscopic measurements demanding thereby the program to cover the excited states. Recently, Noguchi, Hiyama and Koga have shown such extension is possible with the first-principles two-particle Green’s function program.

Fig. 1. Comparison of the calculated (bottom) and the measured (top) XAS spectra for benchmark molecules, acetone (left) and acetic acid (right). Using the first-principles Green’s function scheme, the excitation energy from the oxygen 1s state to the valence states was calculated for the first time without empirical parameters. The calculated excitation energies are in agreement with experiments within the error of 1 %, or 2-5 eV. This demonstrates that theory is able to provide reliable data that will help analyzing the measurements.

The program has been developed by Noguchi on the basis of the Bethe-Salpeter equation (BSE) combined with the GW approximation (GWA) to the self-energy operator, and the problem originated from the extremely large computational requirement has been overcome by the development of the parallel programming. Thus the remaining problem is to test the accuracy of the program by comparing with reference data and convince it to the users of the first-principles program. In this context, the authors focused on recently synthesized nanomaterials, or the fullerene containing alkali metal inside and a carbon material called warped nanographene, which are extraordinarily suitable for the benchmark. By comparing the measured and calculated spectra, reliability of the GW-BSE calculation was shown [1, 2].

More stringent test was done for the core-excitation spectrum of a molecule [3], which so far has been calculated using the density functional theory together with empirical adjustments. By the comparative study, the authors demonstrated that x-ray absorption spectra (XAS) can be reliably assigned fully from first-principles using the program (see Fig. 1).

So far, the program was also applied to other materials such as the firefly liciferin, or the light-emitting compound found in firefly species, to understand the light-emitting mechanism [3]. The GW-BSE program is planned to open to ISSP supercomputer users and is expected to advance our understanding of the excited states of materials.


References
  • [1] Y. Noguchi, O. Sugino, H. Okada, and Y. Matsuo, J. Phys. Chem. C 117, 15362 (2013).
  • [2] Y. Noguchi and O. Sugino, J. Chem. Phys. 142, 064313 (2015).
  • [3] Y. Noguchi, M. Hiyama, H. Akiyama, N. Koga, J. Chem. Phys. 141, 044309 (2014).
  • [4] Y. Noguchi, M. Hiyama, H. Akiyama, Y. Harada, and N. Koga, J. Chem. Theor. Compt. 11, 1668 (2015).
Authors
  • Y. Noguchi, M. Hiyama, and N. Kogaa
  • aNagoya Univerisy